Upon entering the Thieves' Hideout, you'll quickly encounter the Thief Girl, who is initially trapped. After freeing her, she will become your companion for the duration of the dungeon. Your primary objective is to escort her safely through the various rooms and puzzles. She has her own health bar, so be mindful of enemies and environmental hazards. You can command her to wait or follow you using the A button when near her.
Many puzzles in the Thieves' Hideout revolve around her unique abilities. For example, she can crawl through small holes that Link cannot, allowing her to activate switches or open doors from the other side. You'll often need to merge into walls to reach areas she can't, then find a way to reunite. Communication is key; she'll often point out things or give hints. Ensure she is safe before engaging in combat or attempting risky maneuvers, as her well-being is directly tied to your progress in this dungeon.