This is Hirasawa Ui. She is the younger sister of the Light Music Club’s lead guitarist Hirasawa Yui. She is also a TRICK designed to pull in otaku and leave them with more than they anticipated!
Yes Ui is a little sister, and in that sense many fans may be drawn in by her “little sister” appeal. However, that imouto moe is only on the surface, and what lies underneath is a beast from yesteryear, ready to reclaim the throne of fandom, away from the tsundere, away from the “Onii-chan!” spouting imouto, away from the maids, away from the tsundere imouto maids.
Hirasawa Ui is actually a 1980s Childhood Friend Shounen Heroine in the guise of a little sister. She bears few similarites to Kyon’s sister from Haruhi or Cardcaptor Sakura, while her personality is closer to that of Yuria from Hokuto no Ken or Minami from Touch! Closer to her is Mikan from ToLoveRu but Mikan still leans closer to the imouto moe side. Capable, smart, responsible, and always in a position to help those close to her, Ui is the start of a secret plan to bring back the 80s Shounen Heroine, or at the very least the To Heart’s Akari-style childhood friend (as opposed to the To Heart 2 Konomi-style childhood friend).
“Oh!” you might say, “But she’s not the most popular character!” But that’s where they get you. Because she’s not in the main cast, because she’s not at the forefront of her show, she can slowly build up momentum. She is the first of many. She is avantgarde.
Now that you know the truth, you have two choices: embrace it or fight it. Just know that Hirasawa Ui will be there to serve you tea and make sure you’re comfortable.