Mei used to be outgoing, but after her friends betrayed her, she closed herself off completely. She showed up as withdrawn and silent at first—"freakishly gloomy" is how people describe her early on. She's awkward, observant, brutally honest, and has a dry sense of humor that catches people off guard. Her father died of cancer; her mother is still around.
By episode 3, she and Yamato get together (mostly at his insistence). She genuinely cares about him, though expressing those feelings doesn't come naturally since she's still figuring out how to interact with people again. Yamato gives her a blue bracelet and matching rings as relationship symbols, and Mei treasures them—she's said that wearing the bracelet makes her feel like he's with her, which helps her feel grounded and safe.
Content compiled by AnimeList.moe from publicly available sources.