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At first, Merlin does not think highly of Arthur, believing him to be a spoiled prince. This is due to their first meeting, where Merlin called out Arthur for bullying another kid; however, this ultimately leads to Merlin being thrown in prison for a day.
Later, Merlin visits The Great Dragon, who has been imprisoned beneath Camelot by Uther Pendragon. From the dragon, Merlin learns of his destiny to assist Arthur in fulfilling his role as the Once and Future King, uniting Albion.
Their friendship begins to grow after Merlin saves Arthur's life from a vengeful witch. Uther, impressed by Merlin's actions, gives him the job of being Arthur's manservant, much to both of their displeasure.
Throughout the series, Merlin protects Arthur from the foes which try to snuff him out before he fulfills his destiny. However, Merlin does much of this in secret since magic is outlawed in Camelot. Due to this, Arthur believes Merlin is rather useless, but he still thinks of him as a good friend. Their bond, for example, if evident in "The Labyrinth of Gedref", where Arthur must pass a trial to save his people from a magical famine and drought. During this test, he and Merlin are sat at a table, on which rests two goblets. One contains poison and the other does not, and one person can only drink from one goblet. Before Merlin can do anything otherwise, Arthur pours both concoctions into one goblet and drinks it all. His willingness to die for Merlin allows Arthur to pass the test.
At the end of the series, Arthur is fatally wounded, and Merlin finally reveals his ability to use magic. Arthur is surprised and somewhat betrayed, but he ultimately accepts Merlin since he used his magic for good. After Arthur dies, Merlin lays him to rest in the Lake of Avalon, awaiting the foretold day when Albion is in need and he will rise once again.
- Merlin – “How can it be my destiny to protect someone who hates me?”
- Great Dragon – “The half cannot truly hate that which makes it whole. Very soon, you shall learn that.”
- — The Dragon's Call
- “I’ve seen how much [Arthur] needs you. How much you need him. You’re like two sides of the same coin.”
- — Hunith to Merlin; The Moment of Truth
- Arthur – “One thing I tell all my young knights; no man is worth your tears.”
- Merlin – “You’re certainly not.”
- — The Last Dragonlord
- Morgause – “You intrigue me, Merlin. Why does a lowly servant continue to risk everything for Arthur and for Camelot? You know the answer, but you're not telling me. Why? Come on. Time and again you put your life on the line. There must be a reason.”
- Merlin – “I believe in a fair and just land.”
- Morgause – “And you think Arthur will give you that?”
- Merlin – “I know it.”
- Morgause – “And then what? You think you'll be recognised, Merlin? Is that it? All this so, one day, you can be a serving boy to the King? No. There's something more. Something you're not telling me, isn't there?”
- — The Tears of Uther Pendragon, part 1
- “Just... Just hold me, please.”
- — Arthur, to Merlin; Diamond of the Day, part 2
Behind the Scenes
- Julian Murphy – “Well, he's dying, the man he loves is dying, so he's holding him.”
- Katie McGrath – “I think you maybe just confirmed what a lot of people thought.”
- — Commentary track for Diamond of the Day, part 2
Some fans have noticed that Arthur's lips weren't in sync when he said his last words to Merlin. Many believe that it looked a lot more like "I love you", leading many to believe that the moment was later censured.
On AO3, Merthur is the most written ship for both characters. It is the most written ship in the BBC Merlin tag and the fourth most written in the Arthurian Mythology tag.
- Although both characters appear in Once Upon A Time, many people do not ship them in that series, due to their limited interactions, and Arthur being evil in the series.
- The third Tales of Arcadia series arc, Wizards, also has Arthur as a villain. Along with having people of magic being hunted and killed for the powers they hold, like they are in the BBC series, expect for Merlin, Morgana and Merlin's young apprentice Douxie Casperan. Like in the legend that the two characters are based on, Merlin is loyal to Arthur and serves as his advisor. Merlin had originally planned to make the Amulet of Daylight for his King when it was time for them to fight Gummar and his Gumm-Gumms, but after certain events forces them to change their plans and enlist the good trolls for help Arthur has Merlin make the amulet for them. After the Time Map shows Merlin the possible changed timeline that has Arthur dead, the elderly wizard tries to stop this from happening (before the Battle of Killahead Bridge could begin). In the end however Merlin's battle with Morgana prevented him from saving Arthur when he was killed at the Battle of Killahead Bridge, by the hands of the Arcane Order who later used their dark magic to bring Arthur back to life; so he can serve as their Green Knight. Merlin wasn't aware of this until the Green Knight revealed his true identity to him; before the corrupted Arthur uses his own sword to kill the wizard.
- In DreamWorks' Shrek the Third, the relationship between its version of Merlin and Arthur is of teacher and student. With Merlin being a teacher at Arthur's boarding school, until Merlin's lack of control over his own magic got him fired. When Arthur comes across his former teacher on his journey to the Kingdom of Far, Far Away, he and his traveling party stay at his cottage. After they were attacked the next day, Arthur "convinces" Merlin to help them through his magic by teleporting them to their location.
- In Disney's 1963 The Sword in the Stone their bond wasn't only of the one that a teacher would share with his young student, but is also of friendship. As 1963 Merlin uses his magic to teach Arthur things through the eyes of the animals he turns the both of them into, along with helping Arthur with his chores and shares with him the knowledge of wonderful things that Merlin's visions of the future has allowed him to see. When Arthur became King, he is glad to know that Merlin will be there to help him through it.