They're sidekicks who work under Midnight, referring to her as "Mistress Midnight".
They appear to function as a combination hero team and fan club, showing both great concern for Midnight's health following a battle and intense jealousy towards a defeated villain after Midnight mockingly kissed him.
Very little is known about the Midnight Boys group.
After Midnight helped Eraserhead subdue a nameless villain with a Suicide Bomb quirk, the four Midnight Boys appeared on the scene. Following Midnight's instructions for them to hand the villain over to the police, they quickly secured the criminal and dragged him away.