Krillen is Goku's best friend from the time he met him. He becomes one on the weaker characters in a SS dominated universe, but nevertheless he still stays an important character because he is Goku's best friend. It was Krillen's death that triggered Goku to go SSJ1 for the first time. He was a monk, that explains the 6 dots on his head and has one of the coolest moves, the Destructo Disc. He was trained by Kame and by Master Roshi.