Nice, guess the angel is true to its feelings

Actually, the little devil is tempting Marisa to try and steal a sleeping kiss (again) from Reimu. Last time, Reimu woke up before she succeeded. Seems this time, she got doubts, so the little angel took the chance.

I actually laughed out loud, hard.

can anyone tell me what that character between "sleep" and "in" is?

you mean 居, 眠 or 中?

If by "sleep" you mean 眠 and by "in" you mean 中, then that would be り.