12 Beast

12 Beast is a one-shot by Pixiv artist Takemaru "Okayado" Inui that has become an ongoing manga series.

The one-shot follows the adventures of General Eita Touga, his support staff-member Aero, and their army of mercenary monsters as they try to save the world of Live-Earth from the Gigas, massive machines exterminating the inhabitants, the Autana (the series' term for half-human monsters).

With the might of their incredibly diverse troops, strengths complemented and weaknesses covered; the Gigas Slayer, a steel gauntlet that gives Eita superhuman strength and allows him to borrow Aero's power; and his PSP, tablet and a war game for any and all strategic planning, they travel across the land defending the weak and slaying the anti-Autana weapons... provided they don't die of starvation first or stage a mutiny, as said army is perpetually broke, and Eita can't resist helping the needy and poor.

The ongoing tells the story of how they get there: Eita is a high school ninja gamer from present day Earth who'd like nothing more then to keep out of danger, until Aero, a harpy from Live-Earth, shows up to convince him to help protect the Autana, a Gigas hot on her trail.

Eita inadvertently accepts Aero's request after he trashes the Gigas, and so they set out to save Live-Earth, recruiting their army as they go (including Lamiae, Lizard Folk, Centaurs, Satyrs, Minotaurs, Kobolds, Mummies, Dullahan, Golems, Scylla, and Slimes).

... oh yeah, did I mention the abundance of hot monsters and fanservice?