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 first appears to Rinne at the abandoned club building of Sankai High, claiming to have been sent by Tamako to form a contract with him, to which the boy refuses, having no resources to support the helper. He then appears to Sakura, whom he grew attached to and reveals further about Tamako's engagement and Rinne's situation to her, while Rinne finds a letter, discovering the truth about him: Tamako fired him and advised him to get Rinne's support. Eventually, the two form a contract, on the grounds that Rokumon covers his own living expenses.
While supporting his master and struggling together against his poverty, he would go through various occurrences, which some are significant for him. He lets Sakura ride on top of his head whenever she gets involved in serious situations (and also Tsubasa). He would meet other Black Cat Contractors such as Oboro, Suzu and Kurosu, and participate in events involving Black Cats with them.
Rinne allowed him to take the Black Cat Contractor Ranking Exam, since there’s a new discount on Shinigami Tools for a ranked Black Cat. He got a good score on the Written Test and reached top on the Combat Test, and so graduated to 1st Rank, and he was chosen as the MVC (Most Valuable Cat) and is given a lucky cat piggy bank as a bonus reward.
It is revealed that he had been hiding his savings from Rinne in a cupboard at a tool room at the corner of the school building and made up a ghost story to keep the students away, which eventually lured a ghost teacher from another High School, who causes his secret to be discovered. Rinne forgave him but Rokumon would not give any money.
He accepted to pose as Annette’s mascot in a witch coven, but ends up being an unwilling participant to a fusion magic spell and has his torso turned into a pizza, and it took a lot of trouble to revert the spell.
When the Black Cat Secret Meeting took place with no knowledge of its purpose and Oboro assumed it as a protest rally against the Shinigamis, he felt he did not have any complaints over his master, but after the real purpose of this meeting is discovered, he fought Rinne to get his Scythe to open a giant can, but his master opens it himself, and they both collapse in disgust like everybody else. For being the youngest, he would be entrusted the box containing the records of the Black Cat Secret Meeting until the next century, though he declines it.
Rokumon is usually the one who fetches new equipment for Rinne.
Rokumon doesn't seem to have any special abilities besides transformation and travel between the two worlds. He can change into a large, frightening cat demon and a small kitten. In his demon cat form, he can fly around and allow people to ride on top of his head.
As Rinne's contract cat, Rokumon is required to aid Rinne unconditionally. But even before thet formed a contract, Rokumon has a deep respect for Rinne. Rokumon states that Rinne is "cool" when he acts selflessly, however, Rokumon also thinks Rinne is "lame" whenever he acts childish when receiving money.
Rokumon enjoys being with Sakura and respects her deeply, often perching on her shoulder and even referring to her as "Sakura-sama." This may be because his sense of duty to Rinne carries over to Sakura since he believes Sakura and Rinne would be a good couple.
Rokumon sees Tsubasa as Rinne's love rival, therefore taking a disliking the exorcist. Rokumon dropped the honorific in Tsubasa's name shortly after their meeting.
As the human reincarnation of Rinne's mother, he respects her calling her "Ichigo-sama" and provides her Shinigami tools when she asks.
- “Master Rinne is really a tragic person.” (りんね様は本当にかわいそうなかたです) – Chapter 7
- “This is terrible. Miss Sakura is walking home all friendly with a boy!” (大変だーーーーっ！桜さまが男子と仲良く帰ってる〜〜っ！) - Chapter 20
- “If I could give my flu to that idiot, I’d gladly do it.” (あのバカに風をうつせるものならうれしたい) – Chapter 29, when Tsubasa tells Sakura not to go near Rinne and Rokumon who have a cold
- “The initial B… It’s B for Broke?” (イニシャルBって。。。貧乏のBですか？) - Chapter 78, upon reading the R initial in Sakura’s scarf for Rinne
- “That much for working like a normal person…?” (普通に働くのがそんなに。。。？) – Chapter 84, when Sabato cries blood as he is forced to prepare Ramen
- “I swear I won’t tell. That you took advantage of Miss Sakura not being able to see, to watch her undress.” (ぼく絶対言いません。桜さまが見えないことに着替えをのぞいだなんて。) - Chapter 101
- “Ah, it’s the stench of Master Rinne’s poverty” (あっ、りんね様の貧乏臭だ。) – Chapter 130, after wondering what was the stench he smelled from Kuroida’s treasure box
- “And just why are you on Sakura’s laps?” (そしてなぜ桜さまのヒザのうえにっ！) – Chapter 176, talking to Kuroboshi III
- “I won’t lend any.” (貸しませんよ) - Chapter 268, after Rinne discovers his hidden savings and speaks about costs for the Shinigami tools he used to purify the Ghost Teacher
- “Master Rinne, you’re wrong! Sure, our salary payment arrives late, I can’t eat enough, I have to use my narrow shoulders to go buy stuffs, it’s cold in winter, it’s hot in summer…” (りんね様 それは違います！確かに給料の支払いはメチャクチャ遅いし、満足にご飯も食べられないし、ツケで買い物に行かされて肩身は狭いし、冬も寒いし夏も暑いし。。。) - Chapter 351, when Rinne believes he caused nothing but suffering to Rokumon because of his poverty
- Rokumon is written with the kanji for "six mon", "mon" being an old form of currency. This is a reference to the amount of money (six mon) needed to cross the Sanzu no Kawa, the Japanese version of the River Styx.
- 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.
- The black cat ranking system is most likely a reference to the ranking of fox demons in Inuyasha (making Shippo his obvious counterpart).