Young Macintosh was presented as a suitor for Merida in the beginning of the film. She was not terribly impressed with him, seeing him as a vain spoiled brat. He also seemed to be something of a womanizer, willing to show off for any girl he thought was attractive (including Merida). However, when Merida spoke to her suitors' fathers to convince them to let her and their sons choose their own loves, Macintosh was the first to speak up in her favor, even though it meant speaking up against his father, with whom he was very close.
Macintosh and Merida's relationship is typically represented as a tempestuous one, with both sides being passionate, strong-willed, and hot-blooded. They have a great deal in common, including an interest in athletic pursuits, ironic senses of humor, and good relationships with their parents.
Macintosh also seems to be the standard rival for Merida's hand in crossover fics. He is therefore often cast in a villainous light, as the "bad boy." Merida often breaks up with him before starting a crossover relationship.
Note: only post examples from within the RotBTD fandom.