In Another Day, it is revealed that before the events of the game, Neku had a friend who died. While his name is never revealed, Neku's best friend changed his life and he blamed himself for his 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 he did.
It is unknown whether Neku's friend competed in the Reaper's Game, or what his fate was if he did. Some fans suggest Neku's friend was in fact Joshua, however there is little to no solid proof of this.
There is no available photo for Neku's best friend, as he was never identified in the game.