Merlin and Gaius

"Merlin, you're like a son to me. I never expected such a blessing so late in life." "And you are more than a father to me."

- Gaius and Merlin

Gaius was Merlin's mentor and guardian throughout the series, and as such they shared a very close and familial relationship. They quickly came to view one another as father and son and maintained a very close bond even after Merlin met his real father, Balinor, in the series 2 finale.

Trivia

 * In the Merlin documentary series Secrets &amp; Magic, Richard Wilson stated that Gaius was Hunith's elder brother and therefore Merlin's uncle. However, this relationship was never confirmed or hinted at within the show itself, leaving its canonicity uncertain.
 * One of the few times in the series that Merlin tried to physically attack someone out of anger was when Aredian blackmailed Gaius into confessing to sorcery (2x07).