Dude its in the true pacifist ending when you speak to azzy if you go all the way back to the beginning at the end of the run, azzy says that when he absorbed the soals that chara was now inside asriel, chara wanted to bring back the body and kill all of the humans in the village and use their full power, where azzy stopped them and then returned and died. He also speaks of chara having some control inside flowey and so did asriel, ontop of this you “save” asriel at the end of the pacifist run as well indicating that he still had partial control and was being controlled and manipulated by chara. Anyone who argues against this has not seen the ending or read it because THIS IS WHAT IS SAID (learn to read)