Character Analysis
Cave Man
Merlin's wise teacher and friend lives in a cave just above Maridunum and has been one with nature for so long that he kinda looks more like an element than a person:
Now the old man's veins crawled and knotted on them, distended like worms. But it was his face which held me; it was thin, cavernous almost as a skull, with a high domed forehead and bushy grey eyebrows which came down jutting over eyes where I could see no trace of age at all. (I.5.8)
Wormy hands aside, Galapas winds up being the closest thing to a father figure that young Merlin has at this time in his life. The old man has magic about him, too. He knows about Merlin's talents before the kid even speaks, and he can see far into the future. But it's Galapas' kindness and ability to teach that impresses Merlin:
He was a good teacher […] with Galapas to begin with it was only like listening to a story-teller. He had travelled when young to the other side of the earth, Aethiopia and Greece and Germany and all around the Middle Sea, and seen and learned strange things. (I.6.72)
These stories are a magnet to Merlin. He travels to see his cave friend as often as possible, learning to be a healer and seer—until, that is, he sees something pretty darn awful.
Master Friend
Galapas' death throws Merlin for a loop. He doesn't know what his old friend meant when he said that they would meet again. Merlin's also not sure what his next steps should be: he had been relying on his consultation with Galapas for reliable news of the kingdom.
But even more unsettling than all of this is the fact that Merlin doesn't know if he has apprenticed long enough with Galapas to have power on his own. He finds himself in a tight situation before Vortigern, feeling like he can't magic his way out of a paper bag, when this panicky thought enters his head: "I remembered suddenly that Galapas was dead. Perhaps, I thought, the power had only come from him, and perhaps it had gone with him" (III.7.47).
It doesn't take long for Merlin to understand that Galapas really meant what he'd said on their last night together, when he'd taught Merlin to make the mystical fire that gives visions: "This is all I have left to show you" (I.10.21).
It turns out that Galapas really did know what Merlin would need to succeed in the hostile world around him. That makes him not only a good teacher, but also a valuable friend.
Galapas' Timeline