Welcome!
Just my opinion...
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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.