"Spirit Mages" fight evil threats to humanity by borrowing the power of contracted spirits.

Roak Aleas, a second-year student at Utrea Academy, the training institution for Spirit Mages, possesses skills ranking second in his year. Yet, he has never once summoned a contracted spirit, leading the entire academy to focus on him, evaluating that he "must be hiding his true power." However, as for Roak himself–

"Why don't I summon my contracted spirit? Because I don't have any (—)!"

His strength comes solely from endlessly honing his swordsmanship and martial arts using the only spirit he can contract – a micro-spirit. In reality, he’s always fighting desperately just to stay alive...!? Amidst all this, he gets chosen as a member to explore the highly dangerous "Lunar Ruins"...!?

The Spirit Mage who everyone thinks is hiding his true power actually always fights with everything he's got. He’s not refusing to summon his Contracted Spirits, he just doesn’t have any! — this Spirit Mage is still too strong!?

Source: Novelupdates