Can your dreams be seen outwardly and if so, can Sou remember it? Yes and yes! Feel free to ping me about what he's trying to do and what he'd get from it.
Can he use your character's innermost fears or desires to control/kill/violate him/her? Giotto actually has some canon resistance/ability to sense effects influencing him. It's possible, just difficult, and triggering the Dying Will state has a nasty tendency to cancel things out/make him hyper-aware.
Can he call upon your character at any given time to be used as a soldier? It'd be difficult.
By the powers invested by the Dark Forces of Nature, I now pronounce you a puppet. Agree? Y/N ....depends on circumstances, probably difficult?