Girl in the left appears to be marisa, since Imizu likes to draw her with cowboy hats and she has the usual hair/eye color(yellow eyes, blonde hair) The girl on the right is likely Youmu, she has the blue eyes and grey hair he depicts her with. And reisen has the funny ears and extra-large dakka. Odd hair color for her, though.
Well, I notice that Remilia, Marisa, Reimu, Sanae, Reisen, and Sakuya are all mentioned in the info and tags on Pixiv, so out of them, the missing character tag appears to be Sakuya. Along with that, if you look closely, it looks like there's the start of one of Sakuya's braids, and overall it just matches how the artist draws Sakuya, at least in my observation. I'll leave it open to discussion before adding the tag, though.
Also, if there's a tag for different armor sets in Monster Hunter, Sakuya(?) is wearing the Baggi set.
