(Note: I've always had bits of insomnia which pretty much means I have far too much time to lie around and do nothing but obsess/theorize about whatever interests me at the time. With that in mind, please excuse the massive post that follows.
)
*** Lots of SPOILERS follow, so beware ***
So, it seems to be the best way to evaluate this is to think of each of the clues/facts we know and analyze what they could mean and then to review each of the possible answers to "what is Mira?" and determine all the for/against arguments for each answer. I have my own opinion of what Mira could be, but I'll save that for towards the end in an effort to demonstrate how I reached that conclusion. If you just want to see my personal guess, scroll to the bottom.
General Considerations:
Firstly, in order to understand what Mira is, we need to examine what she was like prior to becoming a nightwalker. Now, we don't have tons of information about Mira's human life, but we do have some. Additionally, we can make educated guesses based on the things Mira has demonstrated or done as a nightwalker that are abnormal for nightwalkers. Still, anything about Mira post-change must be taken with a grain of salt since she was fundamentally altered at that point and anything native to Mira could be "filtered" by her new state.
Facts & Clues About Mira:
Mira's Fire Ability: We know quite a lot about this particular clue.
* First, it is an innate, instinct ability and not something she had to learn, although she has learned how to better use it with time. According to Cynnia, this is typical of how light naturi command fire and unlike how witches/warlocks control it. Bori haven't demonstrated this ability, but we haven't seen much of them so can't necessarily rule them out as a potential source.
* Mira uses primarily soul magic to fuel this ability, but can use earth energy when available. Prior to becoming a nightwalker, we don't really know which source she used, but given her unfamiliarity with earth magic when being taught one would assume she used soul magic. However, she could have simply not known enough about magic back when she was human to differentiate (since she was untrained). While Rowe has demonstrated that naturi can use either earth or soul magic, this is atypical of naturi and they primarily use earth magic. Witches and warlocks can also use either magic source. Bori typically use soul magic and haven't demonstrated using earth magic, but given that naturi can use either we can't rule out the possibility that bori can as well.
Mira's Other Magic Abilities:
* Mira learns how to use a barrier spell, like witches/warlocks. Cynnia helps with this, but there isn't much explanation in terms of how she helps, so it's unclear if this ability is specific to witches and warlocks or if naturi can use it as well.
* Mira can lock the naturi in a separate realm. Everything so far indicates that no one else can, or at least
will, do this. It does beg the question though of who locked the bori in their separate realm? Perhaps banishing the bori didn't require as much power... or perhaps witches were willing to help the nightwalkers remove the bori but not the naturi (if the fact that many witches are working for the naturi now is any indication).
* With LaVina's direction, Mira was able to banish Gaizka using unfamiliar magic. LaVina claims to be a witch and perhaps she is, but I also suspect she is... more.
Mira's Power Signature: This is referring to how others perceive and detect her power.
* After her change, her signature behaves and feels like other nightwalkers. She is detectable by other nightwalkers, lycans, Danaus, and witches/warlocks (I believe). She cannot be detected by naturi. I don't recall it ever being explicitly stated, but since Danaus can detect her, I'd assume bori can as well since Danaus gets his powers from a bori.
* Prior to her change, Rowe was able to feel her power, which he described as not naturi, but not quite human. Since naturi cannot seem to detect nighwalkers, Danaus, and presumable bori, that'd mean whatever power he felt was from another source. Assuming we're limiting ourselves to know supernaturals, that means naturi or witch/warlock in origin (since there's nothing to indicate she's lycan). I imagine Rowe would have recognized the power if it was straight naturi or witch in origin though, so something else must at least have influenced it. Somehow Sadira was able to find her as well, though I don't recall if it was ever explicitly explained how she did.
Mira's Detection Ability:
* Normally, Mira can detect other nightwalkers, lycans, witches/warlocks, and Danaus (so likely bori as well). This is typical of other nightwalkers... except for the fact that Mira has on several occasions wondered if the other nightwalkers can't detect Danaus. She's never bothered to ask though so that's currently an unknown.
* When Cynnia stabbed Mira in the heart to complete Mira's circuit of earth magic, Mira was able to feel naturi as well. So far, only two types of beings have been able to do this - naturi and Danaus (again, indicating most likely bori can). It's possible this was just a unique circumstance of Mira's connection with the earth... or it could be that for the first time since becoming a nightwalker, Mira's "original" pre-change abilities were surfacing more strongly due to the closed circuit.
Mira's Earth Affinity:
* As mentioned earlier, Mira can draw on earth magic, which is abnormal for a vampire. She can only do it at swells, but perhaps this is because her nightwalker status acts as a... buffer or filter of sorts that prevents her from feeling the earth and drawing on it except when the power is strong enough to overcome it. In other words, perhaps if she was never changed she'd be able to feel and use earth magic anywhere. We can only hypothesize about this though as there's no mention of what type of magic she uses pre-change or if she had an affinity for earth back then.
* Mira is able to identify and distinguish types of earth even outside for swells (such as the Peruvian dirt with LaVina). She brushes this off as simply due to her having slept in Peruvian dirt but I doubt it's that simple.
Misc Facts & Clues:
* Danaus is very very upset when he learns the truth and his reaction has something to do with his past and upbringing. Thanks to this clue, we can reason that whatever the truth is, it upsets Danaus either because:
A) He has strong black-and-white views and based on these views Mira is something to be abhorred, even more than a nightwalker.
- Witch or naturi heritage wouldn't likely make Danaus view Mira as something even more terrible. Even Bori might not, based on his own unique situation, but it is the more likely of the options.
B) The truth of Mira's heritage causes him to question his strong beliefs.
- Only a bori background would likely cause this, since they are the basis for the angel/demon mythology and possibly further developments could cause him to question the validity of his faith.
C) Mira's heritage is similar in a fashion to Danaus's mother's situation.
- A witch or bori background could likely do this.
* LaVina recognizes something about Mira and knows what she is. This doesn't help much, since all we really know about LaVina is that she claims to be a witch, can do earth magic, and somehow knows how to banish a bori... or at least knows how Mira can banish a bori. LaVina is pretty clearly more than just a witch... she refers to Mira as a 'wayward child' and has knowledge of how to fight and banish the bori. I'm guessing the LaVina is a permanent host of a bori or something of that nature, but in a less master/slave relationship than demonstrated by Gaizka. As mentioned previously, it is entirely possible there are different "factions" of bori and perhaps the 'angel-like' faction is more into symbiotic relationships. If this is the case with LaVina, it'd explain her power, knowledge of the bori, interest in Mira, and the fact that at the end of book 4 she spoke in a voice completely different than her own.
* Rowe calls her "little princess". I guess this
could mean he knows she has naturi royalty ties... but since he never confirms it and he has every reason to drop a bomb like that on Mira, I think it's just a nickname. I guess he could be just holding his cards close to his chest though.
Possible Races:
I doubt a single race is the answer here, but we can at least look look at each and see what does or doesn't fit. If she is some type of hybrid, this could also help us determine which combination(s) are most likely.
Human: If she was human, it'd certainly have been a human with either powers from another race or a specific as-yet-unknown ability or mutation. Potentially she could be a human/<something> hybrid though.
Naturi: There's some pretty good arguments that Mira is at least part naturi, but there are also some strong justifications what this can't work.
* For: Her fire ability is typical of light naturi, as is her affinity for earth magic and the earth. The fact Rowe could feel her power fits with this too. It's possible she was using earth magic pre-change and instinctively started using soul magic post-change instead when earth magic was no longer available. This could also explain her ability to detect naturi when Cynnia helps her complete her circuit with the earth.
* Against: Rowe says she wasn't naturi when he met her in Spain pre-change (though if she is a "hybrid" naturi, that could be the reason). Still, one would think if Rowe knew she was at least part naturi, he'd play that up in his effort to get Mira to question herself and her reasons for keeping the door closed. If Mira was part naturi, her change would have been potentially fatal to her makers, as she points out. Still, perhaps this is another reason three ancients made her and over a period of ten years (instead of just the reasons already mentioned). Additionally, if Mira's powers are naturi in origin, it is likely Sadira wouldn't have felt them pre-change to find her.
* Hybrid?: There are no know naturi hybrids. Cynnia agrees naturi/human pairings are nigh impossible, but I have to wonder if the reason there haven't been any known hybrid births is because of a biological impossibility... or simply because of the naturi attitudes towards humans making such a coupling unlikely, not to mention carrying a child to term. However, if such a hybrid was possible, a naturi/witch hybrid might be more likely - witches have been show to associate with naturi and might be a bit less repugnant in the eyes of some naturi.
Witch:
* For: Can control fire. Can use earth magic and have affinity for the earth. Could also use soul magic. Could possibly explain Mira's other magical abilities. Could possibly be felt by Rowe pre-change (though likely would have been recognized as a witch).
* Against: Can't detect naturi. Would have likely been recognized by Rowe as a witch (unless something else was going on as well).
* Hybrid?: Witches can reproduce normally, so a hybrid is not unlikely.
Bori: There's a lot of unknowns regarding the bori. Most of what we know can really only be based on Danaus's abilities with a few other insights.
* For: Can strongly use soul magic. Can detect naturi. Can detect other bori (Danaus?).
* Against: Wouldn't be detectable by Rowe pre-change. No known fire ability. No known earth ability or affinity. Mira's abilities are different from Danaus's, the only known bori hybrid.
* Hybrid?: Danaus is the only know bori "hybrid". Even Danaus isn't an offspring of a bori though, but a human (or possibly witch-related, based on his mother) child with a bori parasitically attached to his soul. Again, this doesn't necessarily mean a bori hybrid is impossible. For example, bori can possess others (and occasionally as a permanent host) and I'd imagine that another creature possessed by a bori can still reproduce. Perhaps if this happens, the possessed creature can act as a "surrogate" parent and the child gets some bori abilities from the mix. There is another idea of how how a bori hybrid could happen... one that could be particularly upsetting to Danaus: a woman impregnated by a bori still in spirit form. Why would this be particularly upsetting? Think the Virgin Mary and her virginal conception of Christ with the Holy Spirit... talk about an entire faith possibly being shaken if that was really a bori and not God.
My Personal GuessSo, what do I think? My personal guess is a witch/bori hybrid (or something even more complex such as a witch/bori hybrid with naturi far back in her lines).
Why?
The witch background could explain much. It could explain her fire ability as well as her earth affinity. It could explain her potential to wield other magic. It could explain her ability to use both soul and earth magic. Additionally, if one of her parents was a witch, it'd increase the likelihood of a bori being involved (if Danaus is any indication) and also cause Danaus to recall his own past and mother. It'd explain why Rowe was able to feel her power, but not recognize it (since half of it would be foreign bori to him).
The bori background could explain the things not covered by a witch heritage alone. It'd explain why some of her magic is instinctual instead of learned and perhaps why she used soul magic instead of earth. It'd explain why at times Mira was able to feel naturi power and perhaps why she is more attuned to Danaus's power. It could also partially explain why she retained her fire ability after being changed (since bori and nightwalkers have much in common in terms of their power).
If she is a witch/bori hybrid, perhaps her witch abilities have long been repressed due to her change into nightwalker. If my earlier theory of LaVina is correct and she's a "angelic faction" bori host in addition to a witch, it'd explain why LaVina was able to recognize what Mira was, why she referred to her as a 'wayward child', why she knew Mira would be able to banish Gaizka, why she helped them banish Gaizka in the first place, and why she took Mira "home" - to learn about her heritage, to teach Mira to use all her abilities, or perhaps to have Mira take her "rightful place" in some bori or witch hierarchy.
Lastly, if Danaus learned that Mira was half-bori... a true half-bori and not in a parasitic relationship like himself, I imagine it'd be quite a shock. Either because it'd prove she was "half demon" and in his black-and-white mind evil or because it'd be starkly similar to his own situation. Further, if he learned she was somehow "half angel" (ie. the offspring of a bori in an "angelic" faction) it'd certainly make it hard for him to view her as the evil he's long thought all nightwalkers to be... and the fact that "angels" were producing offspring would likely call a lot of his faith into question for reasons I mentioned earlier. Lastly, it'd mean that Mira's situation could strongly remind him of his own mother... a witch who made ties with a bori for power.
Admittedly, my conclusion makes a few assumptions about LaVina and bori, but in my mind it's the best fit so far without completely contradicting things we know. I guess we'll know how close I was (or completely wrong I was) in a few weeks.
Thoughts?
Yea That would up set him in a ripping out his own eyes sort of Oedipus way. 
...I had the same thought. Cynnia made it clear that Naturi and humans cant breed but nothing was said of the Bori. It would certainly make for quite an exquisite creature. I'm still trying to figure out the bori though. Mira said that they are the stuff that the myths of angels and demons are made of. Is there more than one aspect to the bori than the dark soul suckers we have been acquainted with. Is there a light side to that force? ;D I mean, the Naturi are broken into FIVE clans, are the bori : ANGELS & DEMONS? Could she be half angel, half light clan naturi? The riddle remains until Mistress Drake ;D decides. I think she enjoys torturing us a with this puzzle. 
Then there are always Mira's puzzling little references to her guardian angels and Michael and Gabriel and Michael's missing body. And what of Rowe's persistent "little princess" comments? This is frustrating 
Ok! Let me just stop it right there. 