Rokumon is supporting character of Rin-ne and is Rinne's black cat by contract.
Rokumon has three known forms: humanoid, demon, and kitten. In each form, he retains his black fur and green eyes.
His regular humanoid form is a small, black, bipedal cat with a human face. In his demon form, he is a giant cat head with a trail of clouds behind him.
He can also transform into a small adorable kitten. He does this to gain food, since his contract with Rinne does not provide a high income. Rokumon also seems to think the the handouts are better than what Rinne has to offer.
Rokumon is very loyal to both Rinne and Tamako, addressing Rinne as "Rinne-sama". He does what he can to help Rinne with his expenses, but sometimes his methods are a bit extreme; once, he tried to injure a large mass of students just so they would make donations to the weather hutch. His intentions are good, though, however misguided his actions aren't.
He also has the notion that Sakura is Rinne's "woman", and dislikes Tsubasa for obvious reasons.
- Unlike the cute little characters before him (Shippo, Ten-chan) he doesn't show any animosity towards Rinne, instead he shows great respect towards him and likes helping him out.