I don't see much issue with the shield-guy being a non-offensive tank only (for now).

No amount of offensive power will save you if the enemy starts with an overwhelming attack that offensive units can't avert. This is me speculating, but imagine the enemy makes a volcano erupt near the kingdom, lava rivers flowing towards the city. Offensive heroes? Can't do a thing. Naofumi? Activates his by then evolved mega shield to protect an entire town! Even thought smaller scale, a fantastic defender allows his offensive companions to better focus on damage dealing.

Ffs, I could even see some plot twist later on that'd basically: "The 3 offensive heroes failed, the 3rd/final wave was too much - but the Kingdom survived thanks to the shield hero - and thus a new cycle can begin". Stuff like that.

Anyway, my point: having no offensive powers is not an impossible problem.