Mamiya Chiba is the boy who was Mikage's reason for creating the Black Rose Duelists. He is Tokiko Chida's younger brother, and quite ill.

Spoiler

Mamiya and Mikage grew closer together, and when Nemuro Memorial Hall burnt down Mamiya confessed as the arsenist. This may have been to take the blame off of Mikage, but it isn't really specified.

(Source: Wiki)