Strike Witches

Asahi, a boy who loves video games is killed in a traffic accident and ends up in another world. He tries to enjoy the otherworldly adventuring life he's always dreamed of, but his stats are on the level of an NPC's. He is in big trouble and is being chased by a monster when he hears, "Fear not, little brother!" and the monster is killed in one hit by Asahi's older sister, Maya, who followed Asahi to this world. So Maya has acquired some seriously OP skills, but she takes her adoration of her younger brother to extremes. Thus begins their story about an overpowered older sister with a brother complex and a younger brother with the weakest of stats in another world.

One day, humanity was invaded by another world. Having experienced that battle, known as Other World War I, humanity moved to marine floats, built for emergency evacuation purposes, as soon as Other World War II began. It's around that time that Kizuna Hida is summoned by his elder sister and visits the Strategic Defense Academy Ataraxia where he meets Aine Chidorigafuchi, a girl who uses Heart Hybrid Gear Zeros to fight the otherworldly enemies. He has a tough time dealing with her, but when her Gear runs out of energy, the only way to get it back in action is to do something lewd to her!