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 crust 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 MangaUpdates)