The Super Hero (Rise of the Shield Hero)

This is the story of Akio Satō. He lives by himself in a small apartment in Tokyo. One day, while out on a jog, he finds a weird book in a bush. When he reads it, it transports him in a blinding light.
When the light disappears, he finds that he is with 4 other guys. Everyone has a weapon, except for Akio. Instead of a weapon, he has a purple gem attached to his right glove.

What will the new world bring?