Kyouko, on the first impression of her, can come off as a smug, overconfident jerkass. Because, well, she is. Certain of her part in the world and her place in the metaphorical foodchain, Kyouko doesn't take no for an answer when a goal she has in mind gets interrupted. She's quick to relay the points of her own superiority if provoked, even to other puella magi. She doesn't take anyone's crap, and is seems to dislike authority figures by default. She's blunt, rude, and overly crass in terms of language--out of the entire cast, she's the only one who swears, and her language both in Japanese and English is very informal, taking no measures to be polite or correct in grammar. Coming from the background she does, she's a "hardened veteran" per se, and it takes a hell of a lot to crack her, or even get her strained/upset. Her motivations are selfish, and that's really what makes up the majority of her character--her "look out for yourself and no one else" attitude. She doesn't mind resorting to violence when push comes to shove. Or really, even, just when she becomes annoyed, she'll physically lash out at those around her.

She wasn't always like this, however. In fact, the wish that granted her her puella magi abilities was one out of complete selflessness: for people to listen to her father's preaching. They came in droves, boosting her family up from poverty and near-starvation. But through one way or another, Kyouko's father found out about her wish, going mad at the revelation that no one was coming of their own free will, and called her a witch. He killed the rest of the family and himself, leaving Kyouko as the lone survivor. Kyouko lost any semblance of "faith" she had, calling the concept of religion "false hope." She seems to waver on this topic, however, as she has a rosary she's said prayers with in times of crisis, and the one place she's actually seen staying during her on-screen appearances is an abandoned church.

With her family's tragic end hanging over her head, Kyouko became jaded. She views "hero" mindsets as silly and stupid, promoting the "fight for yourself and only for yourself" motto to the "knightly" character Sayaka. Heroism is dead to Kyouko, her mindset being "what happens is what happens, and it doesn't matter if a few dozen people get killed from it." She does what she feels needs to be done, without hesitation. It's stated that because the wording of Kyouko's wish was "to enlighten them", she gained light-based powers in her puella magi form. When her father went berserk and her faith was lost, she physically lost these powers, and hasn't ever gained them back, showing how deeply scarred she actually is. Her family going without food for days on end affected her mental state for the rest of her life, making her obtain and snarf down food whenever she could. She abhors the idea of wasting food, even when Sayaka just throws an apple onto the ground in rejection of Kyouko's offer.

On the inside, though, she's not that bad of a person. She watched out for Sayaka, as Sayaka's wish was to help another person as well, and she didn't want her to suffer the same fate. While at first their relationship was bickering about the nature of helping people, Kyouko quickly grew attached to her, trying her best to prevent Sayaka's inevitable downfall. She felt the pain of being alone, with no one beside you, and for coincidental reasons was the last person to see Sayaka alive before she turned into a witch--right in front of Kyouko. She decided it was better to die there with Sayaka's witch then turn into one herself and die alone, slayed by another puella magi. And so she did, summoning up all of her power, and committing suicide as she blasted away Sayaka's witch with her soul gem and her spear. 

Because being lonely sucks.