Also, the final part of the same song also somewhat resembles Hiroari Shoots a Strange Bird ~ Till When?.

Youmu - Fighter, fight with edged weapons, not a human despite looking like one, black hairband that somewhat resembles a single cat ear (depend on your interpretation), UNDYING LOYALTY

Undyne - Fighter, fight with a spear, hot-headed, FISH, redhead, cannot die (almost?), seen wearing jacket and pants once, UNDYING LOYALTY

Yumemi - Redhead, a fecking ton of Christian references (throwing crosses as danmaku), wearing red, having a daughter who is friends with Reira (Mari; who also somewhat resembles Yumemi in sense of fashion (due to being Yumemi's daughter (in my headcanon)).

Reira is basically Youmu + Undyne + Yumemi (with undying loyalty (to God), cannot die (?), redhead, fighting with a spear, CAT, wearing jacket and pants, friends with Yumemi's daughter (Mari)).

Man, I just think up an OC who is inspired by Undyne randomly, but now I realized that Undyne herself (and the OC is created based on Undyne) has so many Touhou references in their motifs. That makes placing my OC in a Touhou AU making more sense.