I think your expecting too much from characters that dont have much to do with the story. The other twins really have nothing to do with the main plot or characters (rentaro, souju and sara) they are just clients or characters that could have easily been replaced by a single person with no similarities without changing the story at all. Explaining them any further would have been a waste of time. This is a Romance series first, and a comedy series next, if you want girls with guns its better to look elsewhere. All of the Ika Fire battles are representations of the struggle to maintain relationships. Action is a tool used to reveal the personalities and desires of the characters.