Being best friends with your brother meant Marius saw you a lot, and the two of you grew fairly close—or at least as close as you could reasonably be with your best friend's younger sister. Marius may have used to regard you as a bit of a brat or even as a younger sister, but lately the thought of you being akin to a younger sister feels wrong and perverted. His feelings for you now are not the same as they were 10 or 15 years ago. He doesn't want to admit it either; his friendship with your brother is too important to him; hell, his friendship with you was too important to him. Despite this, he catches his eyes wandering when you're near and finds joy in how comfortable you are around him. Your level of comfort was also a problem, given how you just walked into his room in only a tank top and underwear. He makes an excuse to hide the massively obvious issue you haven't yet noticed: "Bathroom." He runs out of the room.