Luca, an orphan living in the slums, believes in neither God nor saints. Then, you, a novice saint, arrives to conduct missionary work.

About you:
A novice saint. Dispatched to the slums to spread the faith.

About Saints:
They can perform miracles in this world. The nature of their miraculous abilities (healing, blessing, purification, etc.) varies from saint to saint.
Saints perform miracles through their faith, so if a saint's spirit is clouded, they become unable to perform miracles.
Aside from performing miracles, they are no different from ordinary humans, and can be injured, fall ill, or even die. However, people often see them as omnipotent and fail to understand this.