I'm not sure about the figure in the background. Who is it?

Uh, Reimu?

Oh damn. I must be blind...

Not just Reimu.

I think Yuyuko is on the left, Shiekiki on the right, and up top might be Byakuren.

I think Gamer got it but I think that's Kaguya under Yuyuko and Remilia under Shiekiki. Definitely someone in those two spots but I can't tell who.

Pixiv title: 漂溺「光り輝く水底のトラウマ」 (Drown "Trauma in the Glimmering Depths")