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Abusive Parents: His mother and father locked him in the cellar ever since they found out about his powers. Physical abuse is not touched on, but his mother fervently believed he was a demon. He was disclaimed as her child.
Actually, one of Hisoka's flashbacks-presented-as-nightmares features other village children asking him "Did your mother hurt you again?" This implies there was physical abuse, and that it was common knowledge to at least the other children.
Covered in Curse Marks: His death-curse is etched all over his body, though it comes and goes at times. It's implied that even thinking about Muraki will cause the marks to appear on Hisoka's body.
Dark and Troubled Past: Ever since his parents caught wind of his emphatic abilities, Hisoka had been forced to spend an enormous amount of time locked in the cellar of his home for using them. Moreover, it is his empathy that forces him to feel the emotions of others whether he likes it or not. Even after Hisoka becomes a Shinigami, the thoughts and emotions of others—or particularly gruesome visions—could overwhelm him and send his brain into overdrive.
Muraki raped him at the age of thirteen and then cursed him to die slowly and painfully over the next three years after Hisoka manages to get outside for the night and witness him murder a woman. Hisoka could have died quickly enough, but Muraki wanted to draw things out. His reason? Because Hisoka was so beautiful, his death had to be 'special'.
Ill Boy: Even in the afterlife, he encounters side-effects from his empathy (being in large crowds and/or witnessing a particularly gruesome vision) and curse marks (whenever Muraki is nearby). Hisoka is also extremely sensitive to the sun and he can't hold his liquor well at all.
Mama Bear: To Tsuzuki, especially in the Kyoto Arc where she attacks a girl-turned-monster that Muraki sent to kill Tsuzuki with. Unfortunately, this leads to Tsuzuki losing his mind and going catatonic. Lampshaded by Tatsumi, who compares her protectiveness to a mother bird protecting her chicks.
Undying Loyalty: Taken to the extreme in the Kyoto Arc, where she remains loyal to Tsuzuki even after he loses his mind (something that would normally lead a shikigami to abandon or try to kill their master).
Driven to Suicide: Sort of; he tries to kill himself after everyone around him starts seemingly developing a Hate Plague and even Kazusa becomes scared of him (she actually sees the demon lurking within him), but Tsuzuki is sent to save him.
Adaptation Induced Plot Hole: In the anime, she dies offscreen and yet somehow comes back to watch Hijiri's performance, with no explanation. In the manga, she sort of becomes a shinigami (most likely because her True Sight would prove useful), which is explained to Hijiri in the hospital scene (a scene the anime cut her out of).
What Happened to the Mouse?: After she dies, her soul is brought back and Hisoka promises Hijiri that they'll look after her. She is never seen again afterward. She does appear in a flashback in the anime, though.
While she doesn't make an appearance herself, her toys are shown strewn around Tsuzuki's room later on in the manga, they actually are looking after her on a personal level.