If you have "The ‘DENPA’ MEN: They Came By Wave” or "The ‘DENPA’ MEN 2: Beyond the Waves” installed on your 3DS transfer your hero over to use in this game.
Also, as you continue through the story, you'll be able to buy a facility called the Summoning Shop, where you can bring other Denpa Men over to this game.
It costs in-game money to summon these Denpa Men, but they'll come with slightly higher stats than usual!