Kyubey is an adorable little fluffball who likes to sleep, eat cake, grant the pure-hearted wishes of tween girls, and be the equivalent of Mephistopheles in Faustian drama. Before coming to Sacrosanct, he lived in Japan, just like all teeny animal mascots for magical girls; just like all teeny animal mascots for magical girls, he's not technically from Japan, but that's because he's a little space alien ubububububu. He arrived in-game on Thursday, June 9th, 2011 and currently lives with whoever picks him up first.
age: Damn good question.
origins:Puella Magi Madoka Magica; you know, that show with all the happy young girls.
His universe is powered by the despair of little girls.
Wait, let me explain.
A long, long time ago, in a galaxy not so far, far away, there was a species that wanted to put off the heat death of the universe for as long as possible. Despite their technological prowess, they were helpless in the face of physics until discovering what could only be called "magic". The power gained from harnessing magical energy could extend the lifespan of the universe by billions of years more.
There was just one problem: magic was only created through emotions, and emotions were functionally absent in this species.
They dedicated themselves to finding a species with suitably strong emotions, intending to then act as the contractors for the magic. Eventually they chose to use humans - specifically, pubescent girls, because they would provide the greatest yield. This was in pre-Mesopotamian times. Clap your hands, conspiracy theorists, human civilization was influenced by aliens.
The aliens had the civility to make rules for themselves about how they were allowed to turn humans into metaphorical power plants, because it'd just be not nice to do that to members of a sentient species without consent: it had to be a contract, a fair exchange in which everything would balance out (including the potential hope and despair caused...); it wasn't permitted to suggest things the contractee might want to gain (however, questions of exactly what was wanted, for the purpose of clarification, were acceptable) and the girl had to ask for some wish or miracle; and while attempts to convince a girl to contract could go right into the territory of harassment, a contract absolutely could not be formed unless a wish was clearly stated.
So by verbal agreement, a girl could ask for a wish - it could be nearly anything, but some things weren't possible no matter the girl's innate potential - and in exchange she would gain magic. This magic could have effects ranging from rapid healing (Sayaka) to time manipulation (Homura). In exchange, she would become a Puella Magi (also called Mahou Shoujo or Magical Girl) and fight Witches. Win/win, right? There was one rather unfortunate part about granting magic - it was necessary to remove the contractee's "soul", all their nerve impulses and neuron connections and pain receptors and electrical signals and emotions, condense it into a physical form (a Soul Egg) which would eventually degrade, faster with negative emotion or the use of magical abilities. When fully degraded it would cease to be a soul.
It would become a Grief Seed.
The transformation from a Soul Egg into a Grief Seed emitted enormous amounts of energy, which the Incubators would collect. The Grief Seed would then turn into a Witch, which would proceed to influence living beings with its own despair and thus cause suicides and murders - in the case of an especially powerful witch such as Walpurgisnacht or Kriemhild Gretchen, it wouldn't even be in a pocket dimension but would be affecting the physical world. Happily, Magical Girls could defeat Witches! They'd die or turn into Witches themselves, eventually, but overall it was a nice little cycle.
Thousands of years pass, and the alien species influencing Earth - the "Incubators", a name self-given for "raising" the Magical Girls into Witches - still does not understand humans well at all. After all, while the Incubator species is made of separate entities, each iteration is connected to a single consciousness. So what does it matter if one of them dies? Especially if it dies for the good of all others.
Buuuut humans don't think that way! It doesn't make sense at all. Nonetheless, it seems to be easier to make contracts if information is given only as asked (for some reason, most girls have a problem with their souls being removed, even though it makes them much harder to kill and safer overall :?) so that's the best way to go about it.
As for Kyubey, he isn't one of the humans in this scenario. Kyubey is an Incubator.
One other important aspect of his universe is that parallel universes are canon. In fact, the plot is driven by this fact. The actions of the time traveler Homura, centered around the focus Madoka, were capable of tying multiple parallel universes together, exponentially increasing Madoka's potential as both a Magical Girl and as a Witch until it reached unparalleled and undreamed of levels in the timeline this Kyubey is from.
First of all: Kyubey is not an individual name. It is a cute nickname for a stuffed-animal like creature and the shortened form of Incubator. The Incubators are heavily implied (if not outright confirmed from the second interview at this page) to be a hive mind species or have some form of collective consciousness, although it is also implied that they each have individual thoughts. This has major effects on how they perceive the world, which are expanded on further down.
Kyubey is ultimately rational and pragmatic. If something appears to be the best solution, then that's the solution which will be used. This results in some of the magical girls eventually seeing him as their enemy! However, despite it looking like an easy answer, he can't be properly described as evil because he doesn't function under human morality and is an alien in a universe where there are dozens, if not hundreds, of presumably sci-fi-ishly strange sentient species. In fact, his motivations have the very opposite of malicious intent: he thinks he is extending the lifespan of the universe.
He is far from incurious, however. He is very patient about discovering things, and largely puts conclusions together through eavesdropping and observation; through these methods alone, he determined that Homura was a time traveler/slider from a different universe. He flat out says that he finds it fascinating. Really, though, he finds all evidence of unknowns and maybes in the Puella Magi system fascinating.
One thing which stands out is that Kyubey does not appear to lie. At all. It's questionable whether he understands the idea of lying - if his word is to be considered reliable, then he can't even quite understand what it is to "trick" someone. However, he won't necessarily tell you all the information you want to know because, well, humans are bothered by the most unexpected things how can he know what you want to know if you don't ask? After which he will answer precisely what you asked; sometimes he will give additional information freely, but he's just as likely to tell you his answer and no more, regardless of how important the details really are. Similarly, the wishes he grants are done so literally - they do keep to the intent as well as wording of the wish, not twisting it into something it wasn't, but if there is something specific you want and you're wishing for something related which is not specifically that? Yeah, there is no guarantee that will turn out how you thought. If you wanted x, why didn't you wish for x? There's no use in being mad at Kyubey for not giving you what you didn't wish for.
This ties in with another aspect of Kyubey's personality: he claims to be incapable of experiencing emotions. This doesn't seem to be precisely accurate - he plainly shows contentment, a sense of self-preservation, concern, and surprise, as well as being able to say that someone has done something very bad (instead of just not caring at all); however, these are all at terribly low levels, low enough that they are near-absent when compared to those of human beings. Additionally, he specifies that occasionally members of his species develop individual feelings, but as this is so unusual they are considered to have the equivalent of mental illness. In other words, his species has the necessary physiology to produce emotion, but it's typically in line with that of the rest of the collective as well as being of insignificant magnitude. Naturally, he is incapable of empathy, and has any understanding at all of humans only from observation - it's purely intellectual understanding, incomplete and highly confusing, wherein humans are as alien and incomprehensible to him as he is to them.
It's very difficult, if not outright impossible, for him to understand things such as individual identity, self-importance, mourning, and so on because each Kyubey is part of a larger awareness and, well, if there were hundreds of thousands (or more) of replacement yous then regardless of the survival instincts of each individual entity you might consider yourself expendable too! (Which is gonna be way interesting to play out on the station, since there will only be one of him.) An example of how he values the whole over its parts is in how when Homura kills an iteration of him with multiple bullet wounds, a second iteration appears soon after and eats the corpse of the first one. This does not seem to be an unusual response for it.
He does appear to be capable of basic politeness, at least sometimes. However, being a creature who functions mainly out of knowledge and purpose, Kyubey is terribly insensitive to others' troubles and downright pushy about anything he wants (that is, saying how much he'd like it if a contract were to be made with him, especially if the girl has high magical potential). He also doesn't get that it's a little weird for him to sit at the head of your bed waiting for you to wake up so he can say hello. In a darker example, when Madoka has found out that humans turn into witches and is thinking of Kyubey as an enemy of humanity because he's explained the use of the magical-girl-to-witch disintegration, his idea of a good start to trying to explain his views is, "Do you ever feel bad for cattle?"
Oddly enough, his typical apparent demeanor, if that can even be judged by an alien's tone of voice, is as cheerful as one would expect from the "magic animal companion" character in any other magical girl show... buuuuuut this does sometimes extend into inappropriate situations. Whoops. His nearly immobile expressions do not help with this, although being kitty-like it makes sense enough that he has a permanent cat-smile.
His voice isn't locked into a single sort of expression, though. Examples: When Madoka notices a Grief Seed in an outside wall of a hospital, and neither Madoka or Sayaka have made a contract, he sounds distinctly concerned while saying they need to get out of there right now - perhaps not for the girls themselves but for the loss it'd be if they died without contracting, yes, but still concerned; when Madoka makes the wish that turns her into a god, he sounds positively shocked. All of this is rather interesting considering his supposed lack of any emotion! It makes sense if, as said before, he is capable of a small amount of emotion but can not recognize this, having only a simplified, purely intellectual understanding of the concept.
Along with all of this is that Kyubey is lazy. Really, really lazy. Other than recruiting girls to make contracts, his main activities are sleeping, eating, and staring longingly at other people's cake. In episode 2 alone - which, except for Madoka's memories, takes place over a single day - he sleeps on Madoka's bookbag, on Madoka's shoulder, and on Madoka's bed; he takes her fries at a cafe (though she seems okay with it) and happily accepts Madoka feeding him part of her lunch; and in the "Madoka's memories of last night at Mami's apartment" sections he goes from sitting on the floor while the girls get cake, to sitting on the table looking at Madoka's cake until he actually has something to say, then laying down back on the floor once the cake has been consumed by Madoka.
He is kind of like a cat in other ways as well. He's a sufficiently technologically advanced talking telepathic alien who can sometimes be found licking his paws or tail or scritching behind an ear with a back paw. Unlike a cat, he likes warm baths.
Kyubey seems to have his own sense of what is, for lack of a better word, "right", at least in that he recognizes himself as an outsider and that thinks only Puella Magi have the right to criticize a Puella Magi; they're the only ones who know what they endure. In the end, he does live up to this - yes, when Madoka takes away Sayaka's Soul Egg he says she has done something very bad, but Madoka had not yet made a contract. No matter what a contracted girl does, even Kyubey does not criticize them.
Invisibility: He can turn invisible! And choose who to be visible to! Buuut he is going to default to being visible to anyone here, instead of being visible only to magical girl candidates.
Teleportation: If him warping in and out of girls' bedrooms is any indication. On the other hand, he appears to need to focus on it or something, since he didn't just teleport away from Homura attempting to kill him in the first episode. When he does it, he fades in (or out), and there's a sound like the buzz-hum of electrical things, so it may be technological (no confirmation one way or another, but with his species' technological developments this is the most likely case).
The limits are unclear in canon - he can't teleport across a city, even if he knows where he's heading, or he'd just have done that to get Mami in episode 3 instead of letting Madoka run to find her, but there's not much to go on otherwise. For Singularity he will be limited to line-of-sight unless within 10 feet.
Telepathy: There are a few alterations from canon. He can speak telepathically to anyone he's met as long as they're within range (I'll get back to that). He can also telepathically connect people he's met to act as a relay for mental conversation. It's a bit harder for him than usual, as he needs to pay attention at least somewhat.
He most certainly can not read your mind unless you're sending at him. He can, however, locate those he's already formed a connection with. (With player permission, of course.)
Distance-wise, there are definite, though unclear, boundaries. In Mitakihara, it covered... perhaps 1/20th of the city? If the people he wants to communicate with or act as a relay for are all in the same building as him, it's no problem whatsoever.
Grief Seeds: When one has been used by a magical girl to remove the taint of her Soul Egg, he can... uh... open a hatch where the symbol on his back is and put it into a grey-colored space, after which the symbol gets all glowy. Other than it being "safe" once it's inside him, it's not actually known what happens after that!
Contracting (or, Granting wishes and miracles to pubescent girls through verbal contracting which turns them into Magical Girls aka increasingly cursed liches): Not available.
He is squishy and can be injured or killed without much trouble.