Unferth was a hostile warrior in Harry Kim's Beowulf holodeck program who distrusted the Doctor and killed Freya.
Unferth was one of the characters in Harry Kim's holodeck version of the Beowulf epic, a proud and suspicious member of the king's hall. Unlike Freya, he regarded the newcomer who called himself Schweitzer with open contempt and doubted his claims of being able to defeat the creature terrorizing the hall.
In Heroes and Demons, Unferth's hostility toward the Doctor came to a head when he attacked and killed Freya. Meanwhile, the Doctor discovered that the creature, a monster in the story, was really a group of alien photonic lifeforms that had been beamed aboard with protostar samples and were holding the missing crew members.