Also known as Akaten☆Hero! Manga by Nankin Gureko.
Yuji Midorikawa is your typical high school student who has a secret identity: He is Green in the high school battalion unit, Gakusei 5 (G5 for short). G5 is the special unit trained to fight and protect Earth against the invasion of the dreaded Galactic force Gurdark and its sexy queen Mayura. But Green has a problem. He really can't fight, and always ends up hiding behind his other team mates. What ensues is a witty and wild ride as our Imperfect Hero struggles to maintain his reputation as a guardian of earth while keeping up his grade point average in school!