... i wonder how Can Youmu sleep in such a “scary” room exactly..?

It's not really that scary. It's just that Reisen isn't used to it whereas Youmu probably feels the most comfortable there.

Tapper said:

It's not really that scary. It's just that Reisen isn't used to it whereas Youmu probably feels the most comfortable there.

I guess that makes sense, people has likes and dislikes about certain stuff that can be different from other things we know...

In previous pages Youmu was excited to sleep on adult futon. I assume this occassion is rare and she probably sleep on smaller futon besides Yuyuko or something.