Side Couple always got more development than main couple.
I mean, it is hard when a bunch of the possible development for the main couple was expedited in the way they got to know each other.
They are closer than we really expect, the main issue being the Youki's difficulty in gauging his closeness to Cecilia (between friendly acquaintance, friends, confidant and lover). Cecilia has slowly been developing a real affection for him, even some love, but with how weird their dynamic is after how close he was initially acting with her, and how subtly Cecilia has changed her treatment of him (she mostly has changed her expressions and is more confident in the way she treats him, more than any thing she does to/with him), he has severe issues feeling up to the task to confess.