anyways back to chara
We all remember that they say "It's time we erased this world and moved on to the next" But what could the next refer to? they were reluctant to bring the world back after erasing it, so it likely was not an undertale timeline. In other words, it was a deltarune timeline. the knight, it's gonna cause a roaring to destroy the world.
chara could just as easily be trying to destroy the deltarune world as well.
Also one of their last lines is related to the whole deltarune theme of control, and that means there’s a good chance they’re the one that interrupts the vessel creation, and likely uses the soul they obtained to use the vessel themselves.
So here’s my theory, Chara from the genocide timeline travels to the world of deltarune, uses a vessel with their own soul, and then tries to cause the roaring. Boom, the knight solved