Modern writers speculate that the historical Merlin may have been a druid, whereas previously he was thought to be related to various Celtic gods. However, it is more likely that Merlin is a construction of Monmouth's imagination. In Monmouth's version of the legend, and many afterwards, Merlin is the creator of Arthur's reign and a powerful wizard. He is almost inept in organising the union of Uther and Igraine, but the motives of Merlin are rarely divulged. He remains a wise figure who is unable to prevent the inevitable betrayal of Arthur, nor does he seem to make an effort in most accounts. After the incident with the sword in the stone, Merlin seems to have had little influence over Arthur's reign. According to Malory, this is because the sorceress Nimue enchants him and imprisons him for the rest of his life. Monmouth, however, states that Merlin brings the wounded Arthur to Avalon and later goes mad. According to Geoffrey, the name Merlin was applied to the magician because he came from the city of Maridunon (Carmarthen), although later legends maintain that the city was named after Merlin. Of all the characters, Merlin has perhaps been the most successful at capturing the imagination of people.

Artists particularly are drawn to representations of Merlin, and Merlin's ghost is said to haunt Merlin's cave at Tintagel in Cornwall. Evidence of Merlin's popularity can be seen in the recent mini-series "Merlin". Most modern interpretations present Merlin as an almost normal human being who does his best to secure Arthur's reign. This accounts for Merlin's apparent failure to protect King Arthur in later events. However "Merlin" returns to the view of Merlin as a powerful magician.