How do you get Professor Lupin in Lego Harry Potter Years 1 4 Wii?

To unlock Professor Lupin in LEGO Harry Potter Years 1-4 for the Wii, you need to complete the level "The Patronus" in Year 3. After finishing this level, he will become available as a playable character. Make sure to collect enough studs and complete any necessary tasks in the game to access him in free play mode.