How do you unlock Seamus Finnigan on Harry Potter Lego?

To unlock Seamus Finnigan in LEGO Harry Potter, you need to complete the "The Goblet of Fire" story missions. After finishing the level where he appears, you can find him in the Hogwarts grounds. Approach him and use a character with the ability to cast spells to unlock him. Once you've done this, he will be available for selection in free play mode.