Been There, Shaped History: Duncan seems to appear everywhere important in the past four hundred years knowing Lord Byron, and the Dali Lama, having fought in the civil war, and numerous other historical events.
Mr. Fanservice: He fights shirtless several times, and gives a full view of his backside at least once(twice if you count Endgame)
Hair Decorations: He occasionally wears a engraved metal hair clip , and is fond of wearing a decorative button of some kind on his hair tie.
Hammerspace: There was times in the show he pulled his sword out between cuts when he had no place to conceal it.
Happily Adopted: Duncan didn't even know he was a foundling until his first death. After he was banished from the clan, his mother still insisted "It matters not who bore you, you ARE my son!" Even 400 years later, Duncan's recollections of his parents are loving and respectful. He even still wears his clan name as a badge of honor and saves the modern members of his clan from a evil immortal out of a sense of duty, despite them banishing him as a demon.
Honor Before Reason: Duncan has such a strong code of honour that he's walked into things he knows are traps rather than violate it.
Katanas Are Just Better: Played with, it's not outwardly stated, but his sword has broken at least two other blades and chopped a desk in half with no signs of damage. The swords first owner even makes a comment stating how much better it is than his European steel(it cut the blade right off it like a peice of bamboo) and Duncan is pretty amazed by it even though he grew up a warrior and probably handled all kinds of swords up to that point.
Deadpan Snarker: Especially when someone implies his age should make him a wise man.
The Fog of Ages/Time Abyss: Methos isn't even sure how old he is. His memory goes back five thousand years and "before that, it all starts to blur." Joe Dawson describes him as "An Immortal who's so old, he can't remember the time of his birth."
Good Is Not Nice: Fights even dirtier then some of the villains, and even kills a evil female immortal when Duncan's sense of chivalry won't let him finish the job
Heroic Self-Deprecation/Cowardly Lion: Whenever Macleod seeks his advice on matters of honour, Methos always advises him to run away, insisting that he himself wouldn't risk his life for anybody. Yet at the same time he is risking his life to bring Mac back from the Dark Quickening, and duelling immortals on his behalf. Methos talks like a coward, but he doesn't act like one.
Hidden in Plain Sight: Under the alias of Adam, he joins the Watchers, and gets himself assigned to tracking down... himself.
Trickster Mentor: To pretty much everyone. He never teaches a lesson straight out if he can help it.
Underestimating Badassery: Several people have underestimated him because he usually avoids conflict. In one case, a possessive immortal who hates that Methos slept with his slave tracks him down in the present day, using both Joe Dawson and Joe's daughter as bait. Said immortal thinks Methos is a weakling and a coward, and expects to easily beat him. Instead Methos curb stomps him, and the last words the other immortal hears is Methos saying "Just because someone doesn't like to fight doesn't mean they can't." You've also got to remember that this is the man who, as a member of the Four Horsemen, took the moniker of Death.
Also, in the bad timeline shown in the last episodes where Duncan was never born, Methos went on one hell of a Roaring Rampage of Revenge when the Watchers are taken over by Horton, who begins slaughtering immortals and kills Methos' mortal lover in an attempt to get to him. Methos promptly teams back up with his old partner Kronos and begins leading the war against the Watchers. Their actions are described thusly: "They made the Russian Mafia look like choir boys."
What Could Have Been: Methos was slated to die but the producers like what the character brought to the series and he became a recurring character.
Would Hit a Girl: He has been alive for thousands of years before the idea of chivalry, after all.
He might have done this for fun when he was Death of the Horsemen, but in the present? They're gonna need to deserve it a lot.
Fantastic Racism: He believed immortals were a crime against God, and that the prize for being the last was the domination of humanity. So he sought to hunt down and kill every last one.
Manipulative Bastard: The whole time he was hunting immortals, he and his men lead double lives as part of the watchers(who took a vow to only observe immortals and never interfere with them) and used the watchers resources to assist in his mission.
Guttural Growler: Had his throat slashed by Mac in the '20s. He survived; his singing career did not.
Revenge by Proxy: He would kill or frame anyone who is somehow related to Macleod.
Sinister Monk He lived in a monistary where immortals took refuge when weary of the game, and killed out of practice immortals as they left holy ground. The head priest banished him when Duncan caught him in the act.