As a player myself, I would like to encourage another player to add a little nuance to his dark and cringey character. The player in question is young and quite enthusiastic, didn't derail anything in-game, and I don't want to impose anything on him.
How can a player motivate another player to develop their character's personality without being pushy ?
For context, we are playing Curse of Strahd as a group of mostly new players. During session 0, we agreed to run a "good" campaign and to each write a couple paragraphs long backstory.
This player's character is the daughter of a pirate, who taught her to fight. His description of her early life mentioned that "everyone on the ship wanted to {have sex with} her, but she defended herself" and that her father was often angry at her for "damaging" members of his crew. Later, she left her father's boat as an act of teenage rebellion.
Since the start of the campaign, he has taken "trophies" (teeth and tongues) from everything we kill to wear as a necklace. A few sessions in, he started to play his character as increasingly pointlessly mean and confrontational to every NPC we meet. A couple times per session, he separates from the group to do things like steal alcohol from a friendly NPC, or bash in the door of a depressed widow without in-game justification.
At the end of the last session, I talked a bit with him to understand his character's behavior. He described, with details, how actually his character was treated like "less than an object" by her father and how he and his crew raped and tortured her all her life "with nails pulled and stuff", and this why she acts like that.
I'm not against a tragic backstory but this maxing-out darkness, rape and drama to end up with a character that is just plain mean, seems a bit pointless.
He seems eager to play a character with a strong personality and backstory but his efforts are clumsy, and sometimes met with awkwardness at the table. The other players mostly let him do his thing on his own. One tried to stop him in-character a couple of times during the last session.