In fact, I think Anghel being a Sophomore who lived his Freshman year "normally" and then got himself kidnapped fits in just fine with how the other characters perceive him. Between his route and BBL, one gathers that Ryouta doesn't know him personally but at least knows of him, or is aware of his reputation, while Sakuya, Okosan, and Hiyoko have absolutely no clue whatsoever, and everyone else either doesn't meet him or doesn't say anything telling that gives a clear hint one way or the other about whether they knew each other before. So, given the idea that Anghel was there last year and then disappeared....

* Why Ryouta would know about him: he's also a Sophomore, and he's also sane. Obviously they never actually met, but Ryouta is at least perceptive enough to pick up on the reputation and such.
* Why Sakuya would not know about him despite also being a Sophomore: he's a transfer student!
* Why Hiyoko would not know about him despite also being a Sophomore: The phrase "11 Wisdom" springs to mind.
* Why Okosan would not know about him despite also being ... um ... Okosan: Probably too busy running around the track in search of the True Pudding to pay attention to gossip.