The seas and rivers of the world hold some of the darkest secrets...
Honestly of all the champions who we can say to have "dark mysterious pasts" Tahm Kench is one I see benefiting from being an enigma. The mystery of what he is adds to his theme and the whole idea of "Do you trust this guy? Will you make a deal with him? Do you trust someone you know next to nothing about, but still is such a charming, charismatic individual?".
Also as a note, his coat is in fact two coats sewn together. Not only did the rioter who made his kit tweet on this but you can actually see it on the back of his model.
EDIT: I just went and looked at stuff, apparently Tahm is actually a legit demon. A River Demon specifically. Well then, so Tahm's a look in to what Runeterra's demons are like!