This is entirely made up my me, and very likely has no correlation to the lore Riot will release.
When I saw Jhin, I was a bit disappointed, his stunning teasers had me excited, but after supressing the initial disappointment, he looks pretty cool, and his kit looks pretty fun, and I look forward to trying him on the PBE, in the meantime, heres a little crappy something I wrote when I saw how mechanical he was:
He was a demacian soldier, an archer, who got so good with his bow, he got bored, and decided to order a custom rifle from piltover, he became very proficient, becoming an artist of death for the glory of demacia (demacia because of his overall style reminding me of fiora), but one battle, tragedy struck, and he was mutilated in battle, losing both legs and an arm.
As he could no longer serve, he was honorably discharged, and moved to Zaun, where he became a recluse, and as he had naught much more to do, with his one good arm, he painted, and as he had once honed the skills of his rifle, he honed the skills of the brush, and created portraits of immense beauty. But sadly, no one took notice of his practice, his communication with fellow city dwellers was minimal, as they shunned him for being demacian, and he soon just kept to himself and his paintings.
As time passed, he spiraled into madness, as solitude and a brimming imagination and memories of past glory overflowed and mingled, he lost inspiration for flat canvas and paint, he looked back upon the times where foes fell before him like wheat to the farmer's sickle, and deep within grew a terrible thirst, thirst for beauty he could only create with glorious death.
This thirst became his obsession, but he could do naught about it, as a cripple, he barely was able to make it to the market in a wheelchair, and could not slake his thirst.
One evening, as he was drudgingly getting his food, he was happened upon by a man, who was... more than a man, shining steel reflected in the dusk, as he met the eyes of Viktor, and in him, Viktor saw not a cripple, but a flower with a cut stem, and he took interest, bought Jhin's supplies, and escorted him home, where he learned about Jhin's past, and saw the thirst in his eyes, Jhin sought beauty, he sought perfection, and Viktor, he sought perfection himself, just in a different manner.
He made Jhin an offer that seemed to lift the yoke from his shoulders, and without a second thought, he accepted. Viktor helped Jhin regain what he lost, and make improvisations on what he had before, but he did not realize that Jhin had no intent to help him in his glorious evolution, Jhin left Zaun, eager to make good of his skills, and return to the art he truly longed for, art carved in flesh with metal.
Welp, just a thing I came up with, probs 400% false, but an idea, sorry its so long, hope you who took the time to read enjoyed :S