This character is tagged as *菫青石* on Pixiv (yes, with the asterisks), but his name is 雪菫 (Yuki Sumire). He was created by this artist for a pixiv project called ぴくしぶ陰間茶屋・千鈴楼夢語り, the translation of which would be something like "Pixiv Gay Prostitute Tea House: Thousand Bell Tower Dream Story". It's called "PikuKage" for short but it has nothing to do with Pixiv Shadow. The "kage" here is short for kagema.

So you can see why I'm not really sure what to tag it.