In the OVA series, Rumiya can not stand his sister and sometimes takes a rebellious attitude towards her when he thinks she is going too far. When he finds out about Ramia and Biff's plot to crash the Moon into Earth, he quickly warns Misa who temporarily switches sides to join Sammy. Although Misa comes back to Ramia's side after Ramia promises her luxuries in Juraihelm, Rumiya again makes an emotional appeal for her to give up the plan. Flustered by Misa's reaction to his apparent boldness, he brings an end to her involvement finally by transforming her back to Misao.
When Ramia goes missing later in the series, he is overjoyed at first. Rumiya changes his mind when he comes across a holographic recording of his sister expressing concern for him. The recording of Ramia says she was "only kidding" not long after Rumiya had already set out. Rumiya transforms Misao into Misa again, and they end up coming across Hiroshi, who has assumed Ramia's powers and her will to destroy Pretty Sammy. Misa is no match for Hiroshi alone and is defeated. Rumiya awakens from the disaster to be found by his still-amnesiac sister. Determined to put a stop to Hiroshi, who stole his sister's mind, he brings Pixy Misa along to aid Sammy in the middle of her battle with Hiroshi. Ramia's memories are restored right as Misa and Sammy defeat.
Rumiya is more submissive and easily bended towards his sister's will than in the OVA, though he still despises her greatly. In one episode, he remembers his sister's phone number by using the phrase "Ramia is evil, Ramia is a devil" as a mnemonic. Ramia gets frustrated easily and finds her brother an easy target to take out those frustrations. Thus, Rumiya finds himself at the end of many physical punishments after he comes back at the end of an episode with no progress.
As a resident of Juraihelm, Rumiya likely has magical powers other than the ability to turn into a bird, but if he does, they are never shown (unlike Ryo-Ohki, who briefly uses some specific powers in the OVA and TV series). Rumiya is often not directly involved in the battles between Misa and Sammy, though in a few rare instances gets involved in a scuffle with Ryo-Ohki.
He develops feelings for Misao over the course of the series and is rather desperate for his sister to leave Misao alone when she is finally about to meet her father again. Rumiya feels incredibly guilty over the whole situation when Sasami and Misao have their identities revealed to each other. He attempts to help Misao recover, but is unsuccessful because he can't get any help from Ramia, who is in a kind of lifeless state after Misa's final loss. Sasami is the only one able to get through to Misao.
Rumiya tries to warn Ramia not to get involved with Romio's plan, not wanting any more trouble to befall Misao after she finally became happy. Ramia ignores him and seals him away in a crystal so he can't do anything. She later undoes this when Romio betrays her and rescues him with the last ounce of her magic by sending him to Earth. Rumiya is able to relay the situation effectively to Sammy, Oryo, and the others who are on Earth during the crisis. After Sammy has left, Misao wants to help out and begs Rumiya to allow her to transform into Pixy Misa. He warns her of the dangers and is reluctant at first, but Misao admits that she has accepted Misa as part of herself and know she will no longer cause harm. Rumiya materializes the baton, and Misao transforms into Pixy Misa without Rumiya's hypnotic glare for the first time.
During the battle with Romio on Juraihelm, Rumiya admits to wanting to protect Earth because there is someone there he likes. Misa teases him on that, knowing that he likes Misao. Later, after the battle has concluded, Rumiya is shy when he has to say goodbye to Misao. Ramia gets frustrated with him and ends up pushing him into accidentally kissing Misao on the cheek.