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Felix the Climber is a monkey doll from The Secret Life of Toys.
Felix was Balthazar climbing teacher. They used to climb the bookcase together - except Balthazar had to climb back down because of his fear of heights.
While climbing up the beam, Felix fell behind the radiator and was gone missing for a long time. Neither Penny and Simon, nor the toys knew where he was, until he was found and rescued in the episode Climbers.