The nickname of "The personification of cool" was given to James Dean.
because he was cool
Nicknames are earned. They usually are given by someone who observes your habits or some wild circumstance you were involved in.
Cool Pappa Bell was a player in the old Negro League. A native of Mississippi, Bell was elected by the special Negro League committee to the baseball Hall of Fame in 1977.
Leo the cool lion that's cool
its not
One cool nickname for Fiona could be "Fi." It's short and easy to remember.
skywalker
brantie
cool man
court
I think a cool nickname for Madlyn would be Maddie, or you could even use Lyn as your nickname. Maddie is a more casual nickname while Lyn is a little more formal, so it depends on what type of nickname you want to have.