Wait, isn't that Koishi's hat?

It might be, but that's definitely Sanae.
...well, with a few nonstandard add-ons, anyway.

Explain the joke to me, because I only see a Koishi, not a Sanae.

It's not a joke. The artist tagged it as Sanae, so it is.
It's a Halloween pic, so she's cosplaying as Koishi.
You might note the ornament in her hair as a giveaway.

shouldn't this have the cum-while-penetrated tag?