This guys kick's ass! and he's cute, bonus! ^_^

Hakkai himself: Cho Gono was an orphan, who became a teacher, and eventualy found his older sister (Kanan), and they became lovers, living happily ever after... as you do ^_~... Until one day his village gave his sister/lover to the demon's, and he lost his mind, single-handedly killing 500 villages and then 1000 demon's only to have Kanan kill herself at the end of it all because she was pregnant with a Taboo Child. Cho Gono became a demon himself (the legend of 'If you kill a 1000 demons you become one yourself' was actually true) and near death. But not to worry he get's fixed up by Gojyo, Then nabbed by Sanzo and Goku who then take him up for trial, where Cho Gono dies, and Cho Hakkai is born, weee, he then lives with Gojyo, and they become best friends (an maybe more if your reading between the lines *wink wink, nudge nudge*

He wears 3 silver cuffs on his left ear that act as 'demon limiters', these suppress his demon side (he wear's them mostly to keep from being affected by the 'Demon Wave' that's sending all other demons kinda mad). His demon self is also rather hard, and is distinct by his slightly longer hair and Green vine like tattoo's and his left eye becoming golden (he pulled his right eye out, it's a false one, why he wears a monocle over it is beyond me ^_^). He is also rather hard in his demon form, the proof for this is when he has to take on Goku in his demon form, and despite the fact he almost kills himself, he takes Goku out with him, go Hakkai!!! So despite all the mild manneredness, he's someone you DONT wana piss off ^_^

Hakkai is the level headed one in the group, always with a smile on his face (apart from when Gojyo's not around ^_~) he mediates some kind of peace when everyone's arguing or being bitchy, even Sanzo quells to his charm every now and then. Though behind the smile is a whole different story.
Hakkai also hates demons, so actually being one himself is practically his punishment for his sin's of murder.

His Relationship with the Others:

With Gojyo = They have been best friends ever since Gojyo saved him. Even getting jealous when one of Gojyo's friends turns up one day. (check Gojyo and Hakkai's Burial saga in the Reload #4 manga :P). He also seem's to be more concerned with Gojyo's welfare than any of the others.
It is also mentioned in 'Saiyuki Gaiden' that these two were suspected of being lovers. something that warm's my little yaoi heart ^_~, who's to say they just haven't gotten a clue yet *buwhahahahaha*

With Goku = He treat's Goku almost like one of his pupil's when he was a teacher :P

With Sanzo = It's hard to say who's the leader. Hakkai almost places himself as Sanzo's 2nd in command and they do talk to each other about what's going on.

PS: If you haven't guessed, Hakkai is this web mistress's favourite ^_^.