Turns out she is actually Suimei's cousin/childhood friend whose family ran the dojo where he learned to sword. Their clan has a super secret sword style that is meant for fighting demons and other spooky stuff with invisible swords n shit, and she of course is a very proficient swordperson. Her and Suimei were very close, especially after his Dad died and he was living by himself. She was summoned by the alliance as a hero presumably around the same time as Suimei and the gang were, but did not have any memories aside from her name. Nominally this was due to an accident during the summoning, but there's a slight possibility that it was intentionally done by the Alliance to have greater control over her. Anyways, she initially stays shut in her room scared and confused, but when the demons invade she gets a few glimpses of her memories, which were mainly Suimei (she doesn't remember who he is) saying some typical shonen MC "i'll save everyone!!1!" shit, and resolves to fight (incidentally those words were originally what inspired her to seriously grind her swordsmanship back on Earth). She then proceeds to fuck shit up with her mad skills and superior nipponese mythril katana. She apparently has the weakest blessing from the Goddess out of all the summoned heroes (ie cheat stats/abilities), and still solos one of the demon generals (though it could be seen as him jobbing), so she's pretty strong. I'm also pretty sure she doesn't/can't even use magic, but she still uses qi/fighting spirit to help pull off some sword bs. Eventually Suimei arrives, shenanigans ensue, and she ultimately regains her memories and begins to awaken feelings for Suimei beyond familial ones (it's pretty obv that she was unconsciously lusting for his secret sauce all along, as all cousins do). Also, for all you nerds who read all this shit but aren't accustomed to wincest, iirc, first cousins in Japan can legally marry, so don't think too much about it
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