Loli's relationship with her grandmother. As explained in an episode, Loli loved her grandmother so much, she couldn't bear to leave her when she and her parents had to move overseas due to her father's work and leave her behind. So, at a young age, she chose to stay with her.
Because of her young age, the other students tend to feel protective of Loli. In episode 5, Lily places both of her hands over Tanaka's mouth when she was about to tell her how babies were born, and tells a white lie to preserve her innocence.
The fact that despite Tanaka's mannerisms— and that they rightfully consider her an annoyance — Ota and Robo are still able to put that aside and hang out with her regardless.
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Kohaku asking the main trio to be friends with her sister could've just been an easy way to properly bring Majo into the series. But after her introduction she's most often shown (when she actually attends school that is) walking or eating with the three of them and the group at one point brought her bowling with them meaning they took the request seriously and really do consider Majo to be their friend.
Chapter 77 is Tanaka's Hidden Depths moment, where she (rather unwittingly) changed a whole family's lives for the better by being a jerk. Hime was so moved by how much Tanaka improved her well-being in all forms that the former effectively forms a crush towards Tanaka.
