Why are they unwelcome? Are they known for not being their bills? If they haven't got such a reputation, they seem like folks who know how to spend money, even if they lack originality and order clothes from the same tailor, using the same cloth.
I wasn't expecting such a princess room from Rena. She's so overly serious otherwise. But maybe it makes sense: she tries to maintain an honours student image, but her room is her only solace where she can breath freely, without anyone judging her because of it (because it's her personal space not seen by many).
Of course Arus would feel more relaxed in Rua's company. Personality wise, she's much better than Rena, has been from the beginning. Though that being said, a part of Rua wants to kill Arus, which makes feeling relaxed in her company funny. Still, he's a professional killer belonging to a group of assassins, so maybe traces of killing intent just make him feel like he's at home.