Series: Harry Potter

Left at the altar: Hermione by Ron - but it wasn't him, he was Polyjuice'd by Cormac McLaggen.

Leaves town and refuses to come back ever: Hermione.

What eventually brings them back, if anything? Ron gets a cold.

Trauma that made them leave in the first place, if any? See above.

Dies for no apparent reason: Hagrid.

Cause of death: A Thestral fell on his head.

Becomes an assassin: George Weasley.

Sacrifices himself or herself heroically: Seamus Finnegan.

Character(s) everyone's almost forgotten about: Hagrid, until a Thestral fell on his head.

Joins a cult: Ginny Weasley.

Has a bitter, loveless sexual "relationship": Crookshanks with Mrs. Norris.

Item or place from their teen years that's destroyed: The Three Broomsticks.