Cappy is the ship captain who brings Mina to Tenebrous Isle at the start of the game. He serves as the game’s tutorial character, teaching the player how Plasma Vials — Mina’s main healing item — work before she sets off on her own.
That early lesson is more important than it sounds. Plasma Vials are Mina’s version of an Estus Flask: they only restore the yellow plasma portion of the health bar, which you build up by landing hits on enemies, and the heal itself can take up to three seconds to complete, leaving Mina immobile and exposed. Cappy’s introduction is the player’s first taste of Mina the Hollower’s attack-to-heal loop — learning it on the ship is what keeps you alive once you reach Ossex and the dangers of Tenebrous Isle.