Bo is a non-canon character that was featured in the comic Mortal Kombat published by Malibu Comics. He was introduced as Johnny Cage's personal bodyguard.
He first appeared in chapter two of Battlewave as Liu Kang was being brutally beaten by Goro. Without the distraction provided by Bo, Liu Kang would almost certainly have been killed by Goro.
Bo survived the entire run of the comics. During the course of events, Bo received a broken arm yet, in the Tournament Edition series, he still managed to defeat Reptile.
In Mortal Kombat: Legacy, Johnny had another bodyguard called Shaun Lo. He quit due to Johnny constantly picking fights with random people.