Which escaped prisoner is the main threat in the original Lego Island game?

The escaped prisoner who threatens the island in the original Lego Island is the Brickster.

Released for Windows in 1997, Lego Island follows Pepper Roni, a pizza-delivery character, as he helps protect the island from the Brickster. The game blends driving, exploration, building, and character-based activities in an early open-world-style design.

The Brickster is the central antagonist, not one of the island's ordinary residents. Captain Click is a police officer, Bill Ding is associated with construction, and Rocket Racer is a racing character, so none of those alternatives fits the prison-break storyline.

The character later returned in Lego Island 2: The Brickster's Revenge, released in 2001. That sequel's title itself reflects how strongly the Brickster defined the original game's conflict.