Alteisen is German for 'scrap metal'.
Alteisen is a rifle disguised to look and function as an umbrella, perfectly fitting with her gothic lolita fashion. It appears to be a basic rifle with an umbrella handle that uses its shaft as a runner for the stretcher and canopy. It can be opened and closes, just like any umbrella.
Mirai is capable of both firing bullets empowered with her ninja energy and using it as a melee weapon.