Ummm Assassin from Fate/Zero Tag? since the mask looks like it?

But it has the kanji used by Sin Sacks. Perhaps it's their leader?

Sin sacks either evolved or fused... Better get your weapons ready, or make yourself comfortable on the highest hill you can find.

That is most definitely Assassin. I'm sure they used the SiN Sack kanji because of their facelessness making them have no identity, just like sin sacks.

On the pixiv, all the oni character names are tagged. I'm not saying what I expect is always what I get, but if he intended that to be assassin he would have tagged it. Seeing as it's also not tagged for sin sack on pixiv, I'll use neither here.

Is that fisting i see on Suika?

Astana said:
Is that fisting i see on Suika?

It looks like four fingers, but hard to see.

Now, imagine a doujin like this happening?

OH Look........ I some how happened to have found one: http://exhentai.org/g/444415/d1f8b9121b/