I actually don't feel any pity towards Lucy even though her childhood was one of the worst just cause of how twisted she becomes. I even thought the anime was bad taste(the manga is good though) cas the guy actually kisses her and forgives her when she didn't deserve any forgiveness/affection after all the sht she did to him. Sorry I had to respond to you reply with this. I get really heated up whenever someone brings Lucy up xD. Feel free to ignore me
Almost every characters from Fruits Basket :/
I wouldn't say these are the worst childhoods but they do make you pity the characters:
One piece: Most characters from One Piece that get their childhood story told (there are some boring one like Usopp's but there are some really good one's like Sanji's actual full story(not just the story with the cook), Corazon's story, Trafalgar's story, Robin's story, etc)
Love in the Mask: Hyun Bin has story that makes you wish for her best
Gakuen Alice: Mikan's and her Mom's life really require you to have a tissue box with you
Ze: One of the kids that has a kami is very pitiful due to his childhood
I'm only mentioning those that have a childhood that make you pity them in the end. I didn't include any characters that had a rough childhood and then grew up to b so twisted that they weren't pitiable anymore (like Doflamingo from One Piece)
Damn ya'll really made it hard for me to choose one to take the crown.
I guess I'll choose One Piece and Love in the mask as one of the worst past ever, I haven't read berserk to judge it but I'll be sure to read it next!
I can't choose only one so here are some that I've read that the people here didn't mention and made me feel really sad and sympathetic for the protagonists (mostly the main ones):
Arima from KareKano (Also another important side-character had a shitty childhood)
Shoko from Koe No Katachi ->actually the beginning of the manga was very unpleasant and made you hate that one person who made Shoko's life a nightmare.
Subaru from Subaru ->Her depressed past made her become a strong and independent young woman.
Stalker (manhwa) -As the title indicates, it's about a stalker who happens to be our protagonist of the story but he's not that kind of stalker you'd expect. And in the story, the author reveals bit by bit his childhood as a student and his struggles.
Shion from Shion no Ou - This is sth. you'd NEVER want to experience
Acid Town - It's a yaoi manga and the story is very well written and basically almost all the characters in the story makes your heart break.
Mars: The female and male main leads have suffered really shocking events.
Shingeki no Kyojin: There's not much to say, basically almost everyone needs some therapy. :(
Looking for the most pitiful character ever, like his childhood needs to be absolute trash that you actually feel super bad for them.