Story highlights "I didn't enjoy doing it," Rowling writes of Lupin's death

It's the third time she's used May anniversary of fictional Battle of Hogwarts to mourn

(CNN) In what's become an annual tradition for "Harry Potter" author J.K. Rowling, she took the occasion of the fictional anniversary of the "Battle of Hogwarts" Monday to mourn the death of a character.

This year, it was Remus Lupin, the werewolf wizard who died fighting the evil Lord Voldemort and his minions.

"Arthur lived, so Lupin had to die," Rowling tweeted, referring to another character, Arthur Weasley, who survived the fight.

"I'm sorry. I didn't enjoy doing it.The only time my editor ever saw me cry was over the fate of Teddy," she wrote, referring to Lupin's infant son with fellow wizard Nymphadora Tonks.

Rowling killed off both in the climactic battle to defeat Voldemort, leaving Teddy orphaned.