Merlin's evolution is also super interesting and goes all over the place. He was added in the 12th century based on a Celtic character:
Merlin first appears in literature in the History of the Kings of Britain by the Welsh cleric Geoffrey of Monmouth (c. 1100 - c. 1155 CE) ... Geoffrey Latinized Myrddin as Merlinus because the strict Latinization of Merdinus would have associated the name with the Latin merdus (feces, dung).
The character originated in Welsh folklore featuring the wildman-in-the-woods motif, a half-savage who lived on the fringe of civilization possessing great power and wisdom.
The most famous Welsh wildman was Myrddin Wyllt (“Myrddin the Wild”), a 6th-century CE bard who was said to have gone insane after a particularly bloody battle and retreated to the woods. Geoffrey wrote a short work, The Prophecies of Merlin (c. 1130 CE), prior to his famous history but this Merlin has nothing to do with the wizard of the Arthurian Legends. After the success of his History, Geoffrey wrote another work, The Life of Merlin (c. 1150 CE), in which the main character exhibits magical and prophetic skills but this work is unconnected to the Arthurian Legends. (https://www.worldhistory.org/Merlin/)
When Geoffrey did later insert him into the Arthurian story, there's a king being advised by his wizards to use Merlin's blood (who at the time was a child of supernatural birth with no father) to sprinkle on the foundation of a tower to ensure the foundation would be sound. And then Merlin flexes by outwitting the wizards and proves them to be lying, tells the king how to fix his tower, makes some prophecies and goes on to build Stonehenge. (This might actually be a pretty neat storygame of its own.)
The original Welsh wildman Myrddin I also should mention is believed to be a possible evolution of the Welsh god Gwydion (name means "Born of Trees) some of whose adventures with his nephew Lleu might be parallels to Merlin and Arthur's relation. Gwydion is a magician, trickster, and bard. (Of interest to some, might be that when I was collecting this stuff back up again, I found a pretty lengthy article connecting him to the god Pan. Haven't read it all yet so I won't go into it here.)
Gwydion's backstory is also kind of hilarious. The first paragraph of the wiki entry:
Gilfaethwy, nephew to the Venedotian king, Math fab Mathonwy, becomes obsessed with his uncle's virgin foot-holder, Goewin. His brother Gwydion conspires to start a war between the north and the south. To this end, Gwydion employs his magic powers to steal a number of otherworldly pigs from the Demetian king, Pryderi, who retaliates by marching on Gwynedd. During the war, while Math is distracted, the brothers return home and Gilfaethwy rapes Goewin.
And more detail here:
https://mythopedia.com/topics/gwydion
Upon discovering Gilfaethwy’s rape of Goewin, Math punished Gilfaethwy and Gwydion by turning them into mating animals. In turn, the two brothers were transformed into elk, cows, and wolves. During the course of each transformation, Gilfaethwy was forced to bear a child.
So you see, the moral is that Arthurian legend CAN be interesting, you just need to add bestality and gay incest! ^_^
Oh, and there's also this 6th century bard Taliesin who has had his various legends tied to Merlin or other parts of Arthurian legend, that's kind of interesting too but I've rambled enough, you guys can use Google on your own.