Utsuho was born in the Hell of Blazing Fires, where, somehow, she met and became good friends with Rin Kaenbyou. Eventually, the two were adopted by Satori Komeiji when the latter founded the Ancient City, and Utsuho has lived happily with them ever since.
Then one day, while she was monitoring the fading Hell of Blazing Fires, a small, glowing red orb descended from above and a voice sounded in her head: by eating this, she would fulfill the wishes of both the underground and the surface world. So she ate the orb, and when she did, she felt someone enter her body. The transformation was painful; a blazing eye tore itself out of her chest, heavy concrete congealed over her right leg, and a rod hotter than anything she had ever felt covered her right arm.
When she came to, she felt more powerful than she had ever felt in her life. And the first thing she did was reignite the Hell of Blazing Fires, creating a new sun and rupturing the earth, filling the Old Hell with lava flows and burning flames once more. It was back the way she remembered it, the way it was when she was a child, when the Yamas abandoned it for the new Hell. And then, she thought, why not expand the Hell of Blazing Fires to the surface world as well? What good did the surface people ever do? All she knew was that they hated Satori and Koishi, her beloved masters, and from what she heard from the evil spirits, they were sinful and did bad things.
Mad from pain, shock, and sudden power, she began preparing to destroy the surface and turn it into a new sun. However, the geyser that came from her newfound power called the attention of Reimu and Marisa, who quickly beat her out of her madness. It was then that she discovered that it was Kanako Yasaka, the god on Youkai Mountain, who gave her her power and caused her all this grief. Nevertheless, she grudgingly joined the Moriya Shrine's project to build a nuclear reactor to eventually power all of Gensokyo, as she was really the only one who could fully control it.
Utsuho is somewhat childlike. She's often quite cheerful, and is extremely kind-hearted to those she cares about. She has the highest devotion possible to her owner, Satori Komeiji, and her friend, Rin Kaenbyou. However, ever since she ate the Yatagarasu and gained her powers, she's been a little off in the head. She often loses control when fighting, not stopping until she thinks she's won, which often leads to the burning destruction of the surrounding area. She would likely have already made several more attempts to satisfy the voice in the back of her head telling her to burn the world if she wouldn't in the process break a promise she made to Satori after the Evil Spirit Geyser Incident. In fact, her dedication to Satori and Rin is the only thing currently keeping her in check. Aside from this, she's never been the sharpest of tacks, leading her to occasionally overlook the obvious, and she doesn't know much about the surface world (though she's learning). However, eating the Yatagarasu gave her an incredible base of knowledge regarding nuclear fusion and nuclear fission, so just ask her about either of those if you have a couple of hours to waste.
Three Legs of the Yatagarasu
When Utsuho ate the Yatagarasu, she gained its three legs which provided her power. The first is her left leg, the "Foot of Fission," which has several glowing orbs orbiting it like electrons to an atom and which gives her power over nuclear fission. The second is her right leg, the "Foot of Fusion," which is encased from the shin down in concrete and which gives her power over nuclear fusion. This is her most cumbersome feature, as it makes her walk with a limp and she sometimes trips over it. The third is the "Third Leg" that covers her right forearm. This is a control rod that doubles as an arm cannon, and it is the only one of the three legs that Utsuho can summon at will. This allows her to control her powers, and if it were ever to break it's possible that her body would become a living time bomb that could easily destroy half of Gensokyo.
Voice Sample: http://www.youtube.com/watch?v=WNUJ4jnyxBM
Satori Komeiji ~ To say that Satori is one of the people Utsuho cares most about in the world would be a gross understatement. The hell raven almost literally lives for her master, treating her like a mother. The last thing Utsuho would ever want to do would be to harm Satori in any way, and she treats the small youkai's word as law.
Rin Kaenbyou ~ Utsuho's best friend in the world. The kasha is like a big sister to Utsuho, constantly making sure the hell raven acts dignified in public but always ready to play once work is done. If Utsuho were ever to lose both Satori and Rin, she would be devastated. She would feel that she had nothing left to live for, and would give in to her destructive desires and once more attempt to burn the entire world.
Yuugi Hoshiguma ~ A good friend of Utsuho's. She feels that Yuugi is a fun person to be around, and appreciates the support the oni has given throughout all the hard times Utsuho and her family have gone through.
Parsee Mizuhashi ~ Another friend of Utsuho's. Utsuho understands Parsee's curse and tries to help the bridge princess overcome it as best she can. Like with Yuugi, she primarily is friendly to Parsee because of how supportive the latter has been to her own family.
Reimu Hakurei and Marisa Kirisame ~ Utsuho is generally friendly with the two of them, mainly because she's grateful to them for stopping her during the Evil Spirit Geyser Incident. She partially feels like she owes them for that, as she knows that if she hadn't been stopped she could have hurt everyone she loved.
Sanae Kochiya ~ Utsuho is on good terms with Sanae, being the only member of the Moriya Shrine to earn that respect from her. The fact that the shrine maiden gave her a fascinating book about the outside world doesn't hurt either.
Kyou Nigatsu & Sora ~ They've only met a couple of times, but Utsuho finds Kyou and Sora to be fascinating. She particularly loves the clockwork hell raven Kyou gave her for Christmas.
Suwako Moriya ~ Utsuho doesn't really feel much of anything for Suwako. She doesn't dislike her, but (probably due to proxy with Kanako) she doesn't really like the goddess either.
Cirno ~ The ice fairy of the Misty Lake is really nothing more than a severe annoyance for Utsuho. Like with Tenshi, they usually get into a fight when they meet, usually ending with Utsuho winning or Cirno managing to stall the nuclear hell raven long enough to make an escape. However, unlike Tenshi, Cirno has managed to do one or two things to earn a slight amount of respect from the hell raven.
Tenshi Hinanai ~ The two are bitter rivals, incapable of acting civil with each other because their egos are both too big. Anytime they meet, explosions both literal and metaphorical ensue.
Akane Chiyoko ~ While Akane hasn't really demonstrated anything worthy of Utsuho's dislike, the hell raven can't help but feel that Akane is trying to replace her or Rin somehow. It's just new pet jealousy, but it doesn't seem to show any sign of abating, no matter how friendly Akane acts.
Kanako Yasaka ~ She destroyed Utsuho's life, warped her body, broke her mind, and made her become something she probably never wanted to be in the first place, all for the purpose of bringing a new kind of energy to Gensokyo. Is it any real wonder that Utsuho hates Kanako more than anyone else in the world?