When you see it...

...Cirno?

Lower middle-right, folks. Two of those fireflies are NOT fireflies. My guess? Tewi.

Mm, none of the tags appear to be for Tewi, but I think I did see one for Reisen ("永遠亭"?) Changing tags, but feel free to change it back.

I cannot think of why Reisen, the one character in the entire cast herd that is stated to be closest to 100% sane, would be creeping around at night like she's freaking Slenderman. That's more Tewi's speed, due to her trickster nature.