Gaius is the tritagonist in the BBC series Merlin. He is the Court Physician of Camelot and has been for the last twenty-seven to twenty-nine years, since the birth of Arthur Pendragon. Additionally, he has a great knowledge of sorcery and the legends associated with it, having studied and practised it for many years before the Great Purge. He is Merlin's mentor and friend, and one of the few people who know of his magical abilities.
Gaius believed that basic problems should be solved by science. He was an excellent physician which has been proven on many occasions. However, Edwin Muirden claimed his methods were "outdated", though this was possibly a lie to give Uther reason to appoint him as Court physician instead.
Gaius had a very broad theoretical knowledge of magic but when he used magic in Season 1, he initially forgot the words (The Poisoned Chalice) or mispronounced the words (A Remedy to Cure All Ills). This shows that his magical skills have greatly decreased due to the many years he went without practice. The only successful time Gaius used magic in Season 1 was when he created an antidote for Merlin when he got poisoned by Nimueh.
However Gaius’ magical skills have clearly improved as the series went on. He used fairly strong magic when the goblin took over his body, though this was probably the goblins magic not his own (Goblin's Gold). He later used magic to destroy a box that contained a Manticore, after luring the Manticore out of the box though he had to try three times before he succeeded. However it required powerful magic to destroy a Manticore’s box, showing that Gaius had indeed grown stronger in magic. Gaius was even able to successfully attack Morgause, who was an extremely powerful sorceress, in order to protect Merlin, hitting her with a spell which sent her flying and stunning her long enough for Merlin to seriously injure her, by throwing her into a pillar (Although the fact that he caught her off guard while she was focused on Merlin may have contributed to his victory). Recently, Gaius was captured by Alator, a powerful warrior and sorcerer. Though he was eventually defeated, he resisted Alator's mental torture for quite a while though it is unclear if he did this through magic or willpower. Gaius even magically put out the enchanted ring of fire keeping him captive although it was quickly reactivated by Alator. Gaius used his magic again to save Merlin after Morgana put the young warlock in an enchanted coma. Though he was succesful Gaius was unsure if his old heart could take the strain were he forced to use such powerful magic again.
Gaius also had a talent for keeping secrets; being able to hide a sorcerer, Merlin, right under the nose of Uther Pendragon, smuggle Morgause out of Camelot without Uther knowing until many years later and help Balinor and Alice escape Camelot (the latter individual twice). Gaius was also capable of hiding and avoiding attacks, because he was the only person, apart from Elyan, to escape capture when an immortal army invaded Camelot (The Coming of Arthur).
- Gaius was the first name of Julius Caesar, as well as the first name of Caesar's uncle Marius. It is a Roman praenomen but the odds of someone being called this in fifth or sixth century England (at the time the series presumably takes place)when the Western Roman Empire had already fallen are unlikely.
- Gaius is perhaps based on Blaise, who according to some sources such as Robert de Boron’s epic poem Merlin, was Merlin’s mentor.
- In Secrets and Magic, Gaius is twice referred as Merlin’s uncle. Richard Wilson also said this in an interview, though this relationship has never been mentioned in the series.
- Richard Wilson plays a similar character, Father Simeon, in Shine Limited's shorter-lived other Saturday-night drama, Demons. Both are experts on the prominent theme of their series, Gaius is an expert on magic, and Simeon is an expert on supernatural creatures.
- Gaius is aged around 72 (from the time of Series 3) as he stated in 'Love in the Time of Dragons that a book was given to him by Alice. The incription inside the book states it was given to him on his 50th birthday, by Alice, before the Great Purge, which was 22 years earlier. He is still alive in Series 5, which takes place five years after Series 3, making him 77 years old, which was incredibly ancient for someone who lived in King Arthur's time.