In order to get roomie's ending, you need to choose a bit of bad choices (let's say 4 or 5?) in any route. It's kinda like the "just bros with a Luxiem member" ending. I usually choose bad choices in the beginning of every route so I can balance it out in the end with good choices and get roomie's ending.
(Good luck when you get the bad endings because my heart broke many times when I went through them... T_T)