Merlin looks very similar to his counterpart and wears robes while carrying a staff.
Not much is known of him, other than the fact that he is a wizard.
Similar to his counterpart, he is well-versed in magic and helped Spider-Man leave the alternate world. Also, he is put off by the main universe Spider-Man asking Knight, if he was sure his counterpart wasn't a fool.
Season 3: Web-Warriors
The Spider-Verse: Part 3
When Spider-Man was chasing Green Goblin, he chose to stay and help Spyder-Knight to defeat the Alchemist despite knowing that he'll be trapped in the Medieval Universe. Fortunately, Spyder-Knight knows a way for his counterpart to leave. Spyder-Knight tells his about what he calls science, they call magic and takes him to Merlin.
Spider-Man is surprised by seeing Merlin being real (and being a counterpart to Doctor Strange). Merlin reveals that some residue from the Siege Perilous still resides in the ether and uses a spell to reopen it. Spider-Man thanks them for helping and for the magic, which Merlin questions to Spyder-Knight about his counterpart not being a fool.