Gaius

Gaius is the court physician of Camelot and has been for the last twenty years, since Prince Arthur Pendragon was born.

Life before the series
Prior to becoming court physician Gaius practiced magic with Nimueh. He rose to prominance when the queen died in childbirth and Nimueh failed to save her. She was subsequently banished from the court, becoming an enemy of both King Uther and Gaius. At the same time Uther banished the use of magic in the kingdom and Gaius, unlike Nimueh and many others, stopped practising magic. However, during the Purge, Gaius hid a Dragonlord named Balinor with his friend Hunith. Balinor and Hunith would go on to concieve Merlin. At some point, Gaius also smuggled Morgause, the daughter of Gorlois, out of Camelot and gave her to the high pristesses of the Old Religion.

Merlin's mentor
Gaius is the guardian of Merlin, who was sent to him by his close friend, Hunith. Hunith feared the repercussions Merlin would face should his powers be discovered by Uther or his men. Upon his arrival, Merlin saves Gaius from falling to his death from a balcony in his room using the powers. (The Dragon's Call)

Gaius acts as a parent and a teacher to Merlin. He frequently tries to encourage him in his abilities while, at the same time, advising him to "look at the bigger picture" such as with the plague in Camelot where Gaius tries to make Merlin see that curing one person can have consequences; subsequently Merlin cures Gwen's father, resulting in her being accused of witchcraft. (The Mark of Nimueh)

Uther trusts Gaius's advice frequently when concerning something to do with magic (The Mark of Nimueh, The Poisoned Chalice et al) although at times they fail to see eye to eye. (Lancelot) When Edwin Muirden manipulates Uther into believing Gaius was no longer capable of his job Gaius is removed from his post as court physician. Furthermore Edwin threatens to tell Uther about Merlin practising magic if Gaius tells Uther about his own use of magic. It is only after Edwin's attempt to kill Uther, and Merlin stopping him from attempting to kill Gaius, that he was once again return to his post.

Gaius was later set to be executed for sorcery when Aredian, a witchfinder, accused Merlin of doing so and he took the blame. But Merlin was able to uncover Aredian's treachery and Gaius was freed, giving Uther some very harsh words on his views of magic afterwards.

Gaius was frequently seen advising Merlin or assisting him wherever he could. It was Gaius who told Merlin about his father Balinor. He was seen saving as many lives as he could during the Great Dragons assault on Camelot and later comforted Merlin about Balinor's death telling Merlin "you've still got me".

In season 3, Gaius was possessed by a Goblin and while under the Goblin's control revealed Merlin's magic. Merlin and Gwen later poisoned him to get the Goblin out of his body but managed to give him an antidote before he died. Gaius then told Uther that he was possessed and that Merlin was innocent. However this was only half true as Merlin was the one who released the Goblin in the first place.

Personality
Gaius was sensible and believed in thinking rationally about things rather than rushing to decisions. He was loyal to Uther despite the methods the king used to deal with sorcerers and the use of magic. The only person who Gaius valued more was Merlin, whom he came to love like a son. (A Remedy to Cure All Ills) Gaius also seemed to have known the Great Dragon for at least 20 years (A Remedy to Cure All Ills) and trusted his advice, but when Merlin told him that the Great Dragon cared only for himself, he did not protest suggesting he too had reason to believe that beforehand. But he also knows that there are times when the Dragons' assistance is needed.

Gaius also possessed a dry sense of humour and was one of the few people in Camelot who was brave enough to stand up to Uther Pendragon (The Witchfinder). He frequently scolded Merlin about using magic but knew when using magic was necessary. Gaius was willing to give his life for Merlin which was shown in two episodes.

Many years before the series began, Gaius helped Balinor escape from Camelot despite the fact that if he'd been discovered helping the Dragonlord he would have been executed. Because he helped Balinor, the Dragonlord felt he owed Gaius a debt that he had to repay and referred to him as a good man. Gaius was also extremely dedicated to his job as Court Physician and would never give up on a patient no matter how serious the illness or injury.

Merlin
Gaius was very close to Merlin, thinking of him as the son he never had. He was willing to give his life for Merlin which was proven on two occasions. (Le Morte d'Arthur', 'The Witchfinder) Gaius was very proud of Merlin but sometimes scolded him about using magic, unless it was for good reason. He often provided Merlin with advise and comfort and was seen hugging him on several occasions. In return Merlin loved Gaius like a father. They also joked with each other at times.

When Gaius was possessed by a Goblin Merlin poisoned him to get the Goblin out of his body but he and Gwen managed to give him an antidote before he died. Gaius felt no resentment towards Merlin for poisoning him as he understood that it was the only way to get the Goblin out of his body and their relationship remained as close as ever. While waiting for Gaius to recover from the poison, Merlin called him a "stubborn old goat".

Uther
Gaius is a good servant and loyal friend to Uther despite Uther's hatred of magic. He has saved Uther's life many times through his skills as a physician. However after being wrongly accused of magic a furious Uther ordered him to be put to death but Gaius was later proven innocent. When Uther tried to apologise for Gaius' suffering at the hands of the Witchfinder, Aredian, Gaius told him that it was not Aredian who was responsible for his suffering, but Uther himself. He also told him that he wasn't the only one who was wrongly accused of using magic and that not all had been as lucky as him. Gaius was one of the few people who could speak to Uther however he wanted and though he was usually polite to the king he would sometimes stand up to him and point out his mistakes. (The Witchfinder)

Morgana
Gaius has treated Morgana for many years and she trusted him greatly, sometimes going to him after having a nightmare. Gaius realised that Morgana's dreams were visions of the future, though he tried to pursuade himself otherwise. He frequently gave Morgana remedies to try and get rid of her nightmares but they didn't have much effect. But despite his caring for Morgana, Gaius agreed with Merlin's choice to poison her when she was responsible for a spell that was affecting all of Camelot, saying that he feared Morgana chose to use her gift for evil.

Hunith
Gaius seems to have a very good relationship with Hunith, Merlin's mother. She entrusted him the training and well being of her son by sending him to live with Gaius. He helped Hunith by hiding her lover, the Dragonlord Balinor from the wrath of Uther during the Great Purge. Hunith also seeks help from Gaius when Nimueh almost kills her.

Kilgharrah
Gaius had known Kilgharrah for at least twenty years and on at least one occasion he visited the Dragon for advice. Gaius respected Kilgharrah's power and wisdom but they weren't friends and Gaius was probably aware that Kilgharrah was desperate for his freedom.

Abilities
Gaius believes that basic problems should be solved by science. He is 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 has a very broad theoretical knowledge of magic but has little practical skill. The two times he has been seen using magic, he forgot the words (The Poisoned Chalice) or mispronounced the words (A Remedy to Cure All Ills). This suggests that his skills with magic have greatly decreased due to the many years he went without practice. Gaius used fairly strong magic when the goblin took over his body, though this was the goblins magic not his own.(Goblin's Gold)

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 escape Camelot.