Yukino founded and runs the Service Club, though she's the only member until Hachiman shows up. She's got the brains and looks to back up her reputation as the "Ice-cold Beauty"—a nickname that fits her distant, unfriendly vibe perfectly.
What really defines her though is this warped sense of duty she has. She genuinely believes it's her job as someone superior to help out all the "lost lambs" struggling through life. It's basically noblesse oblige with a Yukino twist. She doesn't hide her disdain when she thinks others are being naive—especially Yui—and won't hesitate to call it out.
That said, Yui's not bothered by any of it and still calls her "Yukinon" with genuine affection, which is probably the closest thing Yukino has to an actual friendship at the start of the series.
Content compiled by AnimeList.moe from publicly available sources.
