
WITH WHO IS THE FIFTH ONE *O* ???

They are together. A poly pile

Ok I guess i'll just have to explain:
The five gay friends refer to these pictures: http://www.mangago.zone/read-manga/sayonara_futaribocchi_renai_shinsho/nbt/1f761ed998c2ed69/Ch1/16/
and
http://www.mangago.zone/read-manga/sayonara_futaribocchi_renai_shinsho/nbt/1f761ed998c2ed69/Ch1/21/
1. Sayonara Futaribocchi Renai Shinsho: we have Shun and Yuuta
2. Escape Journey : we have Naoto and Taichi
3. Koi to wa Baka de Aru Koto da : we have Sayama and his older boyfriend Maki (who is not shown as part of their gang of friends)
So all five friends are gay and in lovey dovey relationships. ヾ(☆▽☆)