Berenice Gabrielli is a Quincy and Stern Ritter with the designation "Q" serving in the Vandenreich.
She's best known for her brief, fatal encounter with Captain Kenpachi Zaraki during the invasion's opening moves. According to Zaraki himself, Berenice was one of three Stern Ritter who confronted him—though her chances didn't last long. While she talked extensively about her abilities (partly because Zaraki's ears were still ringing from Jerome Guizbatt's earlier roars), she never actually got to demonstrate them. Zaraki slit her throat mid-explanation. Afterward, he carried her corpse over his shoulders along with the others before dumping them unceremoniously in front of Juhabach.
Her actual ability remains a mystery since it was never put into action, but according to Zaraki's account of what she claimed, it would've made him question everything about himself. That's really all anyone knows about what it did.
Content compiled by AnimeList.moe from publicly available sources.