In Another Day, it is revealed that before the events of the game, Neku had a friend who died. While their name is never revealed, Neku's best friend changed his life and he blamed himself for their death, which is implicitly why he became so antisocial.
It is unknown whether Neku's friend existed in the main universe, or just Another Day's. However, when Rhyme is Erased, Neku thinks "This is just like that time", suggesting they did.
It is unknown whether Neku's friend competed in the Reaper's Game, or what their fate was if they did. There is also no available photos for Neku's best friend, as they were never identified in the game.