Answer:
Friendly
Prefix A: Fee
Root A: Friend
Suffix A: "not harmful" or "suitable for"
Definition A: kind and pleasant.
Archenemy
Prefix A: Arch
Root A: Arch
Suffix A: Arch
Definition A: A person who is extremely hostile or opposed to someone or something.
I don't really know the first three are kinda weird but these were the answers i got.