I feel this image is qualified for the 'what' tag.

it's not tagged Touhou or Marisa on Pixiv, but I guess it looks like her. The one on the left could be a few people...Reimu with her hair cut, Renko or Aya. Or not a Touhou character at all, of course.

I think it's one of those Touhou stuffs in NicoNico. I don't feel like looking it up now though.

not sure is this guy is slender man or a kind of nostalgia critic ghost cosplayer

SHUN0 is correct. The image is related to Kiki_(kate_rr)'s “Role-Playing Game Replay” video series in NicoNico:

“Opoona and Paranoiac Yukkuris”

• NicoNico MyList: http://www.nicovideo.jp/mylist/32195608
• Nicopedia article
• Pixiv encyclopedia article (JP)

• Game: PARANOIA
• UV (Game Master): Opoona (Yaruopoona)
• Player 1: Yukkuri Reimu
• Troubleshooter: REIMU-R-YUK-*
• Player 2: Yukkuri Marisa
• Troubleshooter: MARY-R-YUK-*
• Player 3: Kimeemaru
• Troubleshooter: KIME-R-RCP-*
• Player 4: Yaranaio
• Troubleshooter: NAIO-R-VIP-*