Creating a unique Nickname isn’t as hard as it sounds if you think about it. Each character is unique and you only need one quirk or thing that makes them special.
Exp: My oldest Character Regan has the nickname Butterfly. Sounds simple, but there’s a whole backstory with her grandpa and blue butterflies. (same reason basically that I’m called butterfly by summy).
you can go for something like the appearance or a playful version of their name(Reg for Regan, or Kitty for Charlia (Westeria RP) who is a lynx hybrid) but sometimes you can turn a fun moment into a nickname. (Exp: Forky for someone who pokes you to say hi (because you use a fork to poke food))
In the end a nickname is something that suits the person that gives it and is often a sign of being close to each other. Whatever nickname you go for, as long as it’s something that links to only them, then it doesn’t matter what it is!
@RPers, I hope this helps a little by creating unique nicknames for your character.