Angel is an alternate costume for Devil, but is a separate character with her own endings. Not much is known about her, except that she is the opposite of Devil. Angel first appeared in without any storyline explanation, save for that she lies with Devil over Kazuya's soul. Angel represents the good version of Kazuya's Devil Gene. Fans seem to speculate her origin as being due to Jun Kazama's influence during their meet in the second tournament. It's unknown what happened to Angel since she was not seen after Tekken 2.
Angel comes to Kazuya Mishima through his relationship with Jun Kazama. She is the only thing stopping Devil from completly controlling Kazuya.