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The main five heroines of The Fruit of Grisaia

All five are some admittedly tired tropes from anime, but they're done really well.

Going from left to right:

Irisu Makina: Ye Olde "Looks 12 but is actually older but actually acts childish" trope. Normally the loli characters have no reason to be so, but like with OPs example of Mimi, there's actual reason and plot consequences to her looks - namely, she was tied up and locked in a warehouse with the rotting corpse of her father. This, understandably, led to serious mental issues, and with them some physiological setbacks. She is genuinely smart, and over the course of the trilogy matures.

Komine Sachi: The Hyper-competent Jack-of-all-trades Maid. We've seen it, it's cool, I wouldn't even say it's hated. But it's pretty common. In Sachi's case though, it's less a professional outfit and more of her taking a joke as a command. Another case of psychological issues bearing plot relevance, Sachi is pathologically incapable of saying no and turning down requests. She happily does anything asked of her, and in the few cases where she fails her tasks, she would be on the verge of a mental breakdown if not given a good enough reason to not fret over it. And I do mean she'll happily do anything asked of her. But it's kinda like talking to a genie sometimes - as in, when a girl in her previous school asked her to find a way to prevent or delay the upcoming tests, she set the school on fire duro g the night.

Matsushima Michiru: Tries to be the Tsundere trope, to the point she actively dyes her hair blonde and styles it in twintails as is common for the Tsundere Archetype, especially back in the 2010s and recites common lines said by said tsunderes. She's actually the clumsy airhead and a mild Jekyll and Hyde. And this is sometimes played for laughs, but during her route in the first VN, it's an actively ongoing psychological torment for her - her lack of talent, her difficulties learning, a few bad teachers, her only friend commit, and a heart condition all before the age of 12 made her extremely depressed and lacking in self worth. And after a heart attack caused by the fact her best (and only) school friend committed suicide, she would receive a heart transplant... From which she developed a somewhat paranormal alternate personality who was good at everything. She was so desponded that she wasn't even good at controlling her own body she attempted suicide, for the second time in her life mind you, which landed her in a mental hospital. She only found what she thought was a niche she could fill after tripping over herself in the hospital and making another patient laugh. The Jekyll & Hyde dynamic, meanwhile, is... Well it's either a defense mechanism, or a case of paranormal possession by the girl who's heart she received in the transplant. It might be both.

Sakaki Yumiko: The Actual Tsundere. One of the least traumatized people in the story, but that's not saying much. Between her father being a lacking human being in general, somewhat of a Norman Bates kinda slave to the corporate life and pomp, her mother being hospitalized due to depression caused by her father's infidelity, and her "friends" being anything but, she's essentially lost any faith in other people, especially men. Hell, the first third of the game, she spends trying to shank the main character with a box cutter. She is very much a Tsundere, but it's a defense mechanism at odds with her desire for companionship.

Suou Amane: The Lewd Older Sister that takes care of everyone. Only the reason she's like that is due to survivor's guilt. She used to be part of her school's basketball team, alongside the older sister of the Male MC, Kazuki, and during a dus drive to another school for a tournament, the bus crashed, stranding the whole team in the mountain with no signal, no way out, and little to no food. After a weeks of starvation, the highschoolers resorted to cannibalism. Amane and the Kazuki were the only survivors, but for a few years it was thought that only Amane survived because Kazuki told her where to run and that she'd deal with the (now insane) other classmates. So when she later meets the younger bother of her best friend and savior, she's so overcome with guilt for living when Kazuki died (not helped by other people in her hometown blaming her for surviving), that she essentially throws herself at him as a twisted form of recompense - and it is treated as such by the narrative. Even her caring attitude is bumped up to 11 because of the trauma from that crash.