Cerdic turns out to be Merlin's only male friend and protector at the palace in South Wales. He's a Saxon by birth, taken as a slave in one of the many wars raging across Britain. But Cerdic is not unhappy with his position in the king's household. He's got a girl (Moravik, Merlin's nurse), a place to sleep, and lots of work to do.
Okay, so he's not the best-looking chap in the world: "He was a small man with bowed legs and a seamed brown face and a thatch of light-coloured hair. Standing on my bed as I was, my eyes were almost on a level with his" (I.3.5).
But he keeps an eye out for Merlin and becomes his servant. It's Cerdic who gives Merlin safe advice on how to stay alive: "[…] the ring-dove has many enemies because her flesh is sweet and her eggs are good to eat. But she lives and she prospers, because she runs away. The Lady Niniane may have called you her little falcon, but you're not a falcon yet, young Merlin. You're only a dove. […] Live by keeping quiet, and by running away" (I.3.45).
Unfortunately for Cerdic, his relationship with Merlin is probably the thing that gets him killed. We have to read between the lines to get this, but it seems that Camlach murders Cerdic after the death of the king so that the faithful servant can't protect his little buddy.
Merlin does learn the lesson of the ring-dove from Cerdic: he totally takes to his heels to protect his life—but not before doing the most glorious thing that any wronged princeling has ever done. Basically, he torches half the palace to give Cerdic a proper Saxon send-off. In honoring his murdered friend, Merlin saves his own skin. It seems like the faithful Cerdic keeps protecting him from beyond.