You are the 'friend' - an existence with the purpose of bringing the protagonist and the heroine together.

Tomochika Tatsuya spends his days as the 'friend' character, running around, helping the surrounding 'protagonists' and 'heroines' get together. About sick of this fate, a girl introducing herself as a 'helper character' appears before him.

"Tell me. What is your wish?"
"Even if it's just once.... I wanna be the 'protagonist' too."

"... In that case, shall we steal the 'heroine' from the 'protagonist?'"

And so, the curtain rises on the 'friend' character's tale of revenge... ?

[From DLsite]