In the original mythology, Baldur was killed by Hod (his brother) who had been tricked into doing it by Loki. Loki wanted his daughter, Hel , to have a husband but Baldur would only be with her in the after life, and she would stand for no-one else. Loki was sentenced to be tied to a rock in Netherworld underneath a snake (in the book Jormungand) as a punishment for his crime.
In some translations he appears to be an albino, and in a charm to cure a sprain his name is mentioned.
In Runemarks Hel brings Baldur back to life in the body Loki used. When Baldur realizes how he has been brought back he chooses to let Loki have the body, because he would not take a life to save his own.
He does not have a role in Runelight.