While there were plenty of running gags on Merlin, one of the best was how oblivious Arthur was to Merlin's magic. Considering that he spent the most time with the eponymous sorcerer, it was quite hilarious that he didn't seem to realize that Merlin was one of the most powerful beings in the world.
What made the situation even funnier was the fact that other people, including minor characters and villains, were quickly able to pick up on Merlin's talents since he wasn't particularly careful in hiding them. Although Arthur did eventually find out in the show's last episode, he could have easily found out earlier if some of these characters decided to expose the wizard.
Update November 05, 2021: It may have been nearly ten years since Arthur sadly died in Merlin, but it appears the Once and Future King's presence is still around as his stories have influenced many movies. From King Arthur: Legend of the Sword to The Green Knight, viewers have enjoyed seeing Arthur's Camelot flourish as he has forged alliances with an array of sorcerers (including Merlin and Morgana). But could the same results have occurred if the BBC's adaptation followed down the same path? Considering how Arthur reacted to Merlin's powers, he and Merlin would have created the greatest kingdom of all.
Gaius (S1 Ep1)
The uncle and surrogate father of Merlin, Gaius played an important part in the young magician's life from the very beginning. After the pair met in "The Dragon's Call," Gaius first saw Merlin use his magic when he saved him from a dangerous and possible fatal fall. Although Merlin denied using magic, he once again exposed his magic when Gaius tricked him into preventing a bucket of water from spilling off the table.
In the many years that followed, Gaius spent years protecting Merlin's secret - going as far as to put his life on the line for him. He also spent many of those years teaching Merlin how to hone his skills and defeat several powerful foes. Fans adored watching this duo as they always concocted some interesting and hilarious schemes.
Kilgharrah (S1 Ep1)
While Merlin certainly had a strong team behind him that helped him save the day, there was one thing that was usually the brains behind the operation: Kilgharrah. First appearing in "The Dragon's Call," Merlin was surprised to find The Great Dragon hidden beneath the castle and that he knew practically everything about him. It was Kilgharrah who also told Merlin about his great destiny.
Although Kilgharrah and Merlin didn't always see eye to eye, he did play an important part in Merlin's arc. He would often provide Merlin with some wise advice and knowledge about the magical foes he faced. Merlin and Camelot wouldn't have got as far as he did without his help.
Nimueh (S1 Ep3)
There have been plenty of iconic villains in Merlin, but one that proved to be quite memorable was Nimueh, a High Priestess of the Old Religion. While Camelot suffered a lot as a result of her plans, the viewers soon discovered that Nimueh only wanted revenge against Uther after he blamed her for his wife's death and killed all of her friends.
However, she soon realized this would not be as easy as she thought as "The Mark of Nimueh" saw her discover Merlin's powers. Sadly, the viewers only got to witness them have an all-out battle once since Merlin killed her in the last episode of the season. Considering the extent of her powers, it would have been great if the writers had partnered Nimueh with Morgana or found a scene where she and Merlin formed a brief alliance.
Lancelot (S1 Ep5)
Known to be the bravest and most noble knight of all, Lancelot was the only Knight of the Round Table to ever discover Merlin's powers. He first discovered Merlin's magic in "Lancelot" when he witnessed Merlin use an enchantment on his lance, helping him to defeat the Griffin.
Instead of turning him in, Lancelot surprised everyone when he promised to keep Merlin's secret (and he did so until the day he died). It really was a shame that Lancelot was in a limited number of episodes as he was one of the few people who really appreciated Merlin for who he was and genuinely cared about his well-being. Fans would have loved to have seen them pair up on more adventures before Lancelot's untimely death.
Edwin Muirden (S1 Ep6)
Edwin Muirden was a warlock who wanted to avenge his parents' deaths at the hands of Uther. He posed as a physician and claimed he could heal Morgana ("A Remedy to Cure All Ills"). While there had been plenty of iconic villains over the years, Edwin was one of the most intriguing ones. He quickly got Uther on his side and even managed to convince the court to turn on Gaius.
Eventually, Edwin realized that Merlin had magic when he saw him open a box that had been sealed with a spell. Edwin then used this knowledge to blackmail Gaius while also hoping to gain Merlin's allegiance. This was a storyline that could have certainly played out across the series.
Mordred (S1 Ep8)
Although it's not known exactly how Mordred knew about Merlin's real identity, the first time Merlin discovers they have a magical bond is in "The Beginning Of The End" when a young Mordred telepathically called out to him for help.
While Merlin and Mordred didn't exactly have the easiest of relationships, especially since Merlin didn't trust him as he had been warned that Mordred would kill Arthur, Mordred did keep his secret for many years. It did disappoint fans that Merlin's concern with the prophecy got in the way of them building a friendship, as Merlin could have become somewhat of a big brother to Mordred and mentored him in their craft. Sadly, this was not the case and Mordred died after committing several devastating betrayals.
Hunith (S1 Ep10)
Considering that Hunith was Merlin's mother and had a relationship with Balinor, it's obvious that she knew of her son's magic from an early age. However, as Merlin's powers continued to grow, Hunith revealed in a letter that she thought it was best to send him to Gaius in the hopes that he could give Merlin guidance and purpose ("The Dragon Call").
While Hunith only appeared in three episodes, the viewers learned a lot about her relationship with Merlin and Gaius. They could see that she was fiercely loyal and would do anything she could to keep them out of harm's way. She also had a pure and selfless heart too, which allowed viewers to see why Merlin always strived to bring out the good in everyone.
Will (S1 Ep10)
Will was Merlin's childhood friend from Ealdor and the only person, aside from Hunith, who knew of Merlin's powers. The viewers first met him in "The Moment of Truth" when Merlin returns to his village to help him and the other residents protect their home from Kanen's men. Sadly, Will ended up dying in the battle after he saved Arthur.
Although Will made one appearance, fans could tell that he was a great friend to Merlin. He had every opportunity to expose Merlin but didn't because he knew of the good Merlin could do. The fact that Will's last moments also involved protecting Merlin showed that he would always have his friend's back.
Cornelius Sigan (S2 Ep1)
Introduced in "The Curse of Cornelius Sigan," the viewers met Cornelius Sigan as he inhabited the body of the thief, Cedric, who robbed his grave. After he witnesses Merlin perform magic to protect Arthur, he offers to form an alliance with him. However, he proved that he couldn't be trusted when he tried to possess Merlin's body after realizing he was more powerful than him.
Considering that Cornelius was meant to be one of the most powerful sorcerers in the world, it was a little disappointing and anti-climatic that they didn't have a major showdown. Instead, he was just forgotten about and sealed back in the vaults of Camelot. Morgana could have easily used him as an ally and she certainly would have discovered Merlin's identity quicker.
Jonas (S2 Ep5)
The right-hand man of Lady Catrina, Jonas also made his introduction in the season 2 episode "Beauty and the Beast," where he helped her trick Uther into marrying her. Jonas was the first person to discover Merlin's powers as he watched him try to use an enchantment on Lady Catrina.
Jonas was a lot more intelligent than Lady Catrina as he realized the danger they were in when witnessing Merlin's powers. It's also likely that he was the one who organized the plan for her to marry Uther since he was the one that arranged her potions. The only issue was that he didn't seem to realize what exposing Merlin could have done for him or Lady Catrina.
