Headcanon that Chara was always with Frisk. The whole time. Even if they weren't in control, they were there, even in the Pacifist route. They told Frisk any information that you might have looked up on YouTube or Google about the puzzles in the Ruins. They're the reason that, when you looked at the toys in the room Toriel gave you, you thought "Look at all these cool toys! They don't interest you." Because they brought back memories that they didn't want to think about. Idk, that's just me.