He's the Sacrifice Unit of Breathless. He’s the stronger one of the two and since he is Ai's sacrifice he gives her strength with his encouraging words and with his strategies on how to best defeat Soubi.

Once they are ordered to stop trying to retrieve Ritsuka, they talk to him and warn him not to trust Soubi, but are sent back home to finish with their own schooling. They are very close and don’t like to be separated. Their names are on the palms of one of their hands.

(Source: Novellalive and Angel Fire)