Recently transferred to Tarou and Makoto's school from Tokyo, Masayuki Nakajima tries to make friends with both due to an interest in investigating the kidnapping incident. While he was initially ignored by both, they eventually warmed up to him, with the three of them beginning to try to find the circumstances behind the kidnapping incident and the reason behind their O.B.Es.

He tends to be very confident and initially, somewhat arrogant and rude. He has had a fear of heights ever since he bullied someone to die by suicide by jumping off the school roof, and the victim left a message on the blackboard cursing Masayuki. Masayuki unashamedly admits he ran away due to the message. Instead of feeling guilt, he is angry at the student for making him a murderer, a fact he cannot get out of his conscience. He is, however, very determined to overcome this problem and tries extensively, including standing on the edge of the school roof, to cure his fear of heights.

During his spare time, he often plays his head-mounted display virtual reality game. His father is a researcher at Japan Bio-Tech, while his mother appears to spend large amounts of time playing console games, primarily Tetris or something similar. Of the two other boys, Masayuki has a friendlier relationship with Tarou, visiting him in the hospital and joking with him.