Maron is the catulus girl that works at @home cafe along with Hitomi. She usually takes the magic Nekomimic candy made by the Professor to hide her ears and tail. She goes by the name Oguri when she appears as a human, like when she goes to school or does modeling work (and her model name is Kuripon, but only Ayano talks about that.)
Yuki suspected that Maron and Oguri are the same person allllll the way back in chapters 14 and 16, but the rest of the group are airheads and haven't made the connection yet. Yuki has never mentioned her suspicion since then, so for all intents and purposes, no one knows about Maron's dual-life (except Kurumizawa, who seems to understand what is going on.)