Frocin is the "wicked" dwarf who seems to have it in for Tristan and Yseut: it's he who leads Mark to the site of their secret meetings. He also comes up with the scheme to spread flour on the floor between Tristan's bed and Yseut's, hoping Tristan's footprints will betray him. Frocin is an astrologer: he can tell the future by looking at the stars and planets, a skill that helps him evade death when King Mark turns against him.
Frocin's motivations are mysterious: we're not really sure why he's so eager to get Tristan and Yseut in trouble, nor why he decides to throw in his lot with Mark's barons. Maybe Frocin really is just plain evil. This characterization is one Frocin's dwarfism backs up, since medieval audiences thought that a character's outer appearance revealed the state of his soul. By this logic, if Frocin is ugly and deformed, his soul is, too. (What does it mean for Mark to have horse's ears, then?)
His venom doesn't seem to be reserved just for Tristan and Yseut. In a strange episode in Part 6, Frocin betrays the secret that King Mark has horse's ears by telling it to a hawthorn bush while the barons are nearby. Turns out that Frocin's meanness extends beyond gossip: he actually gave Mark those ears in the first place, a fact Mark reveals before promptly cutting off the dwarf's head. Frocin apparently forgot to read the future that day.