Mamiya is a very mysterious character. At times he can seem all sweet and innocent, and at others he can seem like the personification of evil. He never interacts with anyone except for Mikage and his sister Tokiko. He is always seen in the chamber at the bottom floor of Nemuro memorial hall, watching over his black roses.
Along with Mikage, he chooses people to become black rose duellists. Mikage appears to be in control, but it's obvious that Mamiya knows a little bit more than he lets on.
We later find that Mamiya really isn't Mamiya. The boy we come to know as Mamiya is in fact an imposter. The real Mamiya died decades ago. So, who is the imposter, you ask? Well that's a good question. After Mikage's duel with Utena, we shift to Akio's observatory. Akio explains to Mikage what happened and hangs up the phone. He then walks over to Mamiya who is staring out the windows overlooking the academy. Akio asks if he is sympathizing with Mikage. Mamiya remains silent. The camera shifts to a closeup of Akio's face, as he explains that Mikage never existed in the academy in the first place, and that Mamiya has not existed there either. The picture then shifts back to Mamiya. But, Gasp! We no longer see Mamiya standing there, it is Anthy! Now we know why she was always so tired during this saga. She was leading a double life! =)
I truly am sorry for the brief profile of Mamiya, but he really doesn't have that much screen time. He is an important character in this saga, however. I included most of Mamiya's information in Mikage's profile, it's kind of impossible not to. Therefore I really didn't feel like typing all that junk again. So if you want to find out more about him, make sure you read Mikage's profile.