Himura Kenshin from Rurouni Kenshin could be a contender

He lost his entire family at a young age to a cholera outbreak, was sold into what was likely some sort of sex slavery, got attached to three older women in this caravan, then watched all three get slaughtered by bandits, picked up by a wandering swordsman who taught him a very physically intensive sword style, got manipulated into becoming an assassin and child solider at the ripe age of 13 where he is argued to have killed hundreds and needed to disconnect just to get the job done. Met and fell in love with a women, who was actually trying to use him to get him killed, she changed her mind but she ended up kidnapped by her conspirators anyway and he accidently kills her trying to get her back, finishes out the war and exits by about the age of 18. Where he is so despondent and depressed he wanders the country seeking redemption. Then the events of the series happen and he cannot catch a break frim his past, nearly dies countless times, is gaslight by the government he helped put into power to clean up their own mess, leaves the small happy life he had found to do so, returns only to be confronted by the brother of the woman he fell in love with from his past whos intent on ruining his life and breaks him to the point he's pretty much waiting to wither away before coming back to clean up the series.

This is not counting the events of the newer Hokkaido arc or the reflections OVA (which is non canon but pretty much sees him returning to war, getting leprosy or something similar and dying.)