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Miho is...complicated. Short version: Lots of fainting spells and time in the hospital. Long version: She's theIll Girl, the in-universe Trope Maker. Her story, the archetype of a young girl dying from some nameless disease, tugged on the heartstrings of the world so much that it became more "real" than she ever was. She is forced to relive her sickness and "die" over and over again. How long this has been going on is unclear, but long enough that she hates the archetype she created. And just to further complicate things, she's The Chessmaster and the closest thing the comic has to a Big Bad.
Miho: Have you ever fallen in love with a girl because you knew she was going to die?
In-universe, Kotone, the girl Kimiko is voicing in the game Sight, is another Ill Girl. Details are sparse, but apparently she was kept innocent and sheltered her entire life. She's an Expy of Miho, just like every other in-universe Ill Girl. Once we see Miho with long hair, the physical similarities become more clear.
Puppet from the Whateley Universe subverts this through her caustic attitude, plus a learned snobbishness from her upbringing. She definitely has it hard though, making her something of an Iron Woobie.
In Knite, we have a terminally ill Min-Min, as a result of pollution.
Magical 12th Graders: Yeorum is a psychological version during the current story, and in flashbacks she was doted on by her parents for physical problems. She was faking at least some of it for the attention.
Parodied in Ansem Retort, where Axel and Zexion deliberately infect Riku with AIDS to make him the poster child for their charity AIDS Aid.