The Story of Fizz
Fizz is a yordle — or possibly a vastaya, or possibly something else entirely — who appeared in Bilgewater from the deep ocean without explanation or apparent backstory. He moves through the city's canals and harbor waters with amphibious ease, stealing fish directly from the nets and market stalls of people who can never quite catch him and aren't entirely sure what he is. The sea creatures of Bilgewater's waters seem to regard him as a friend or perhaps a colleague.
His trident is enchanted with something ancient — magic from the deep sea that predates Bilgewater by the length of geological time. He uses it with playful precision, as if "playful" and "lethal" are not in tension. He treats everything as a game, including combat, including theft, including the attempts of Bilgewater's guards and bounty hunters to apprehend him. He has never been caught. Most people who try give up somewhere around the point where he laughs at them from underwater.
Nobody knows his actual origin or purpose. This does not appear to concern him. The ocean is old and full of things that washed up without explanations, and Fizz has found Bilgewater to be a perfectly adequate place to be whatever he is. He steals, he swims, he laughs, and the large thing lurking beneath the harbor's surface that occasionally surfaces when Fizz summons it suggests that his relationship with the deep sea is considerably more significant than his attitude implies.