Now, be a good child and turn back. We don't want to lose you both.
(Chara used buttercups or whatevs it was in Undertale while making some meal and Asriel ate it, although she didn't know it is not edible, so Asriel got sick badly and humans have experience with ills so she wanted to absorb his soul and leave to help him)
PS: In this take monter's souls can be absorbed by a human being while they are still alive, although it slowly takes their magic and then when they don't have any more, soul essence then they go dust, so you can't absorb a soul for too long. Also after releasing monster's soul if its soul essence was used it will not be able to use magic as well anymore. It also can shorten their life after a big ammount of soul essence used. Since Asriel is a boss monster like Toriel and Asgore which is required to leave, Chara could leave The Underground with it, if it worked the way I just said.