A stowaway (密航者, Mikkō-sha) refers to an individual who has stowed away or hidden on a ship.
There are various reasons why an individual may choose to stow away on a vessel rather than just openly board it - they might wish to travel somewhere or follow someone without anyone knowing, need to escape from someone in a hurry, wish to travel but be unable to do so going by proper procedure, or intend to commit acts of sabotage.
There are various methods which are stowaway might do to board a ship and conceal their entrance. This might include hacking into the ship's security systems to override them or modify information, as Gruier Serenity did with Gonzaemon's ID ring when she first snuck aboard the Bentenmaru . After boarding, a stowaway usually conceals themselves in a section of the ship where they can evade detection.