Welcome!
Just my opinion...
Anything with an X you'll see many, many times. So it'll be easy to replace if you ever want to. And in fact, using these characters for higher-level sacrifices will generally get you better characters. If you can sacrifice 10 3 star characters, you have a better chance of getting 4 or 5 star souls in return.
Anything with a √ or ? are more rare and even if you don't power them up, seeing them again may take a while, but they will reappear eventually. Those with a √ appear less often and are generally worth powering up.
Eddies are Eddies
Viking and Pharaoh are good.. I'd power those up given the others you currently have. DON'T get rid of the default Eddies = Eddie, Wicker Man Eddie, and Soldier Eddie - or you'll
NEVER see them again, Mummy Eddie, Speed of Light Eddie, and Run To the Hills Eddie all appear fairly commonly in Rare Soul drops - especially Mummy and Speed of Light. The only other Eddie - Storm Eddie - can drop, but not very often.