Check Intentions.

"How do i know you wont be the same as Frisk then? the kid who murdered my friends. the one who's body your in."

"i was getting to that. there was nothing more i wanted then the complete happiness of monsters. its why i died and would continue to sacrifice anything and everything to make monsters happy."

"then why appear ONLY now???"

"Because, frisk had to have enough determination for me to fully and permanently possess her."

"So... no more frisk?"

"well, you can CALL me that, if you want, but, nope, no more frisk."

"...What do you plan to do with her body? everyone's dead."

"You cut me off. i was going to say i think i should make it up to you in frisks absence. i can prove it to you."