Chad's backstory and past are rather unknown. As of this moment, the only clues as to his past were shown in a dream in part four. He's thrown out of the dream from some assuredly traumatic event, at least by the expression on his face.
He also appears to share the same mindscape as The Voice as well.
Chad's appearance is quite different from Frisk's. Not only is he dark-skinned and has spiky hair, but the stripes on his shirt replace purple with an even lighter blue.
Chad is a rather headstrong young kid. He's been shown to be able to shake off things that would send many people into a rather large state of shock with a little bit of effort; as shown by his initial silence due to the discovery of the Figments, before quickly adapting to this new environment and brand new species.
As of this moment, there are no existing motifs or songs associated with this character.