Despite his great size and greater strength, Young MacGuffin is a calm, gentle, and very shy lad. He speaks in an uncommon Scottish dialect that is incomprehensible to most, so his father must reiterate on his behalf.
As the oldest son of Lord MacGuffin, Young MacGuffin became one of the three suitors for Princess Merida. During his introduction, he proved his strength by snapping a log with his bare hands, but it was clear that he was uncomfortable being in front of an audience. He had dinner with Merida's two other suitors, Young Macintosh and Wee Dingwall, but excused himself to join King Fergus' bear-hunt through the castle.
Later, during Merida's speech to the Lords about letting their children decide their own fate, Young MacGuffin admitted to his father how unfair he thought the engagement competition was to them and Merida. He took part in (unwittingly) hunting down Queen Elinor in her bear form, and was present when she returned to human form.
At the end of the film, he said goodbye to the other clans, during which he accidently pushed Young Macintosh off the dock.
According to his voice actor, Kevin McKidd, Young MacGuffin was going to be the suitor Merida chose before it was decided that she would remain single. However in an intereview with McKidd, he mentioned that Young MacGuffin did in fact like Merida romantically.
On a related note, in a scene that was later cut, Merida shows interest in him, but was disappointed that she couldn't understand him.
He must have at least one younger brother, because when he's being presented to Merida along with the other two suitors, his dad refers to him as his eldest son.
Young MacGuffin lines are all in a dialect of Scots language known as Doric. Doric is traditionally spoken on the North Eastern Coast of Scotland, between Inverness and Stonehaven. Kevin McKidd, a Scots speaker, requested the use of Doric, these lines were originally intended to be in gibberish:
"Gif he war a wee bit closer A coud lob a caber at him, ye ken" (trans: "If he were a little bit closer I could throw a tree trunk at him, you know.")
"It's jist nae fair makin us ficht for the hand o the quine that disnae want any bit o it. Ken?" (trans: It's just not fair making us fight for the hand of the girl that doesn't want any part of it. You know?")
"Since yer sayin' it an' ah wisn't there t' see it maself, I'll gae ahead and take yer word fer it though." (trans: "Since you are saying it and I wasn't there to see it for myself, I'll go ahead and take your word for it.")