She's nothing but an original character created by the artist. Looks like she is bikini-clad warrior of some sort based off of pages 214-225. I wouldn't mind seeing more a manga by cool food. The character is very pretty and the artwork is quite nice.
Btw, it is not group nor double penetration if you read the dialogue. It's not drawn well, but he says "I'm going to use your anus and pussy alternatively". She never fucks any other guy, it's just his alternative thrusting is represented badly. It's just a vanilla story.