S Rare Soubi No Niau Kanojo

Seko Daiko, a second year high-schooler is an extreme cheapskate. Taking down all the light bulbs in the house, making his siblings eat baked bread crush for breakfast... He would do anything just to save an extra penny.

Daiko was sure he could keep this thrift lifestyle going forever until... The world's fate depends on him spending money on a mobile game?! (From Manga Updates)

Chapter 1
While drawing for game items, Ea got a monster instead but since the game monsters have become real, it came out and possessed her. When Daiko came to her house, she acts unusual and strips to a one piece swimsuit and attacked him under the monster's influence(since girls possessed by monsters in this series all wear one piece swimsuits for some reason).

Chapter 11
A monster sucks out Ea's brain, inflicting reduced intelligence, causing her to do stupid things like cast Berserk on an enemy monster. Though it turned out to be an unfortunate accident since the berserked monster knocked Haruka out of the water, allowing her to switch costumes and easily defeat it.

Chapter 33
In order to investigate where Ea was kidnapped to, Ami infiltrates the monsters by wearing a one piece swimsuit to blend in. Doing so, she successfully located Ea and inform Daiko and his team, along with leaking intel on where they'll attack next. But doing so got the monsters suspicious that there's a spy among them and discovered it was Ami. The monster Patra attacks Ami and wrap her like a mummy and turns her into a zombie, later showing up to fight Haruka under Patra's control.