I can't stop thinking about this Zoe puzzle

I think it's Fiddlestick's?

I read a theory it was his anyways, because his old splash featured a key around his neck and his current one doesnt. shrugs

https://nexus.leagueoflegends.com/en-us/2017/12/ask-riot-ping-and-a-stolen-key/

Most likely Fiddlestick's key. He had a key in his old splash.

because he... Hasa-key!

It was confirmed Fiddlesticks, from one of his oldest splash arts. That's also why he doesn't have one now, Zoe took it.