Radar is Quentin and Ben's other friend, although sidekick might be a more accurate description. He's a human encyclopedia and the go-to tech guy for any research or calculations the group needs done. Radar is called Radar because he used to look like a black version of Gary Burghoff from M*A*S*H, but soon after the nickname stuck, he went through a growth spurt and stopped wearing glasses. His friends decided to keep calling him Radar anyway, though. Also, he has a girlfriend named Angela who gets one scene and his parents collect black Santas.
We don't really get to know much about Radar. He wants to lose his virginity to Angela, but never does, skipping their appointed sex date to help Quentin search for Margo Roth Spiegelman. And… that's about it.
To recap: He's black, and only his girlfriend uses his real name (Marcus), but we never learn his last name, unlike every other teen character in the book (except for his also-black girlfriend), because he's been renamed by his white friends after a white TV character he barely resembles.