Elle.
Kay is bland, and can barely stand on its own. It's
fine as a nickname for
Katharine, I guess - but why would a
Katharine go by
Kay when she could be a
Kathy or
Kit instead?
Elle, while flimsy, does at least have some flair, and I know a lovely one.
masculine list:
https://www.behindthename.com/pnl/191050/124079feminine list:
https://www.behindthename.com/pnl/191050/124080