Jax is an extremily powerfull champ wich race, birthplace and residence still uknown. and he has 3 fingers on his hands. his eyes are 6??? or is he like bard? - a mask and behind the mask a face with... lights??? there are so many questions about him
Jax is the spirit of all hobbos, bent on proving that you do not need money to kill anyone you want. He travels runeterra, searching for snobby rich warriors to best in battle. He is the reminder to respect your opponent, else you may find yourself hit in the head with improvised weaponry.
That's about the only way I can explain the lamp post :P.
The champ.
Holy necro Batman.
It has been all but confirmed that he isn't human and he isn't Void-corrupted, since Void corruption isn't a thing anymore (Retconjuration is the naughtiest form of Naughtimancy). Troll of some kind seems pretty likely at this point.
Considering that we now got a new champ with 
and that Jax is listed as an ionian champion,I'd say it is pretty likely for jax to be either a manifested spirit from ionia(I really doubt that bard is the only one),or a man who got kinda changed by these spirits.
I like the Bard-like idea. Remember that 5 and 1 arrangement that his eyes (glowy mask openings?) are in is also in patches on his outfit.
Actual picture of Jax
http://static.comicvine.com/uploads/original/10/106251/3206348-donatello+(4kids).jpg
He even has 3 fingers.
Easy. He's a level 120 Darkspear monk. :'D
i mean come on. his skin is blue, he's got three fingers and two toes, and his voice is deep like a troll. only difference is the accent, but everyone knows that trolls of a different tribes have different accents. XD
think about it. Tundle is ACTUALLY canonically, a troll. a Frost troll to be exact. trundle also has two toes and three fingers... so why can't Jax be a troll too?
He was Yi's brother all this time
If i know correct the youtuber named Necrit made a theory about what he is. i dont remember but i think he said he was a desert troll like trundle in a way who was affected by the void
He's a ninja turtle.
Jax is a "Kohari" i am really sure he is Saijax in this Story: https://universe.leagueoflegends.com/en_US/story/where-icathia-once-stood/
INB4 incoming Darkin theories
His eyes always reminded me of Yi's goggles. Seeing as they're from the same place, I would say that's what they are.
There's a lot of theories around it. He's supposed to have a mysterious background.
There's a Darkin theory because on his splash art, his lamp has a fire that resembles Aatrox. There's a theory that he is the actual Baron Nashor. There's a theory that he is a troll, and the original wielder of Boneshiver (Trundle's club).
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