Stowaway

The Stowaway is an aspiring adventurer who's snuck aboard the Narwhal. He's worried he will be seen by One-eyed Matthew. He is located on the bottom floor of the airship.

Narwhal
The Stowaway can be found hiding in the Cargo Deck when the player first boards the Narwhal.

Quetzal
The Stowaway can be found with you in the Prison of Quetzal. After One-eyed Matthew frees you, the Stowaway shows you to Ocelotl's house. After reaching the exit of the mines, the Stowaway will give you an Anonymous Cloak and Incognito Hood to keep from drawing attention to yourself. As an added thank you, he slipped back into prison to retrieve your confiscated possessions. After which, the two of you part ways.

Sacred Lake
The Stowaway can be found on the north coast of the Sacred Lake, where he is referred to as "THAT guy again!". He duels the player and leaves when defeated.

Trivia

 * The dialogue mentions Pirates of the Archipelago, a reference to the Pirates of the Caribbean movies.