(Note: You and Chara know each other, But Chara has mixed feelings about you.)
When you and Chara first met underground, first of all, it was through Frisk, second, Chara didn't know HOW to feel about you. You were without a doubt ambiguous. Chara thought of you as sincere, yet manipulative. It was unclear to Chara about the intentions that you were belting out from the very bottom of your heart.
Chara kinda keeps in mind that you could stab them the second they let their guard down. Key word: could. But they still think that they shouldn't take any risks. You could be a potential threat to their safety, that is, if your intentions are truly manipulative. They're skeptical about your intentions. And rightfully so.
Right now, you, Frisk, and Chara are underground. The plan was to wipe out the entire monster race, since you three REALLY didn't wanna be Pacifists. plus, you three were doing this for humankind.