@SekiryuuteiDxD: The Lancer is The Foil for a hero of the closely allied variety. Han Solo to Luke Skywalker. Raphael to Leonardo. Sanosuke to Kenshin. Zoro to Luffy. The Lancer is the one least likely to be a team player. He's the one who sneaks off on his own to advance the team's goals independently. He might be jealous of The Leader, with an attitude of "Why can't I be the leader?" When he finally gets his chance he may well find himself asking himself, "Now what would The Hero do?"
If The Complainer Is Always Wrong and there's a chronic complainer to act as the show's Butt Monkey, it's likely this guy. He's also the one most likely on The Team to go turncoat, and the last one The Hero will suspect. Conversely, if the rest of the members turn their backs on The Hero for some reason, the Lancer may be the only one who sticks by his side. The Hero and The Lancer may also be rivals for a love interest, or one of them will have a cute sister whom the other crushes on, only to have the brother say "My Sister Is Off Limits!" Sometimes The Hero and The Lancer may be love interests to each other.
In the event that The Leader of The Team is unable to lead, The Lancer steps in; unless of course the Number Two is someone else in the group. Sometimes, he's forced to take the position against his will. Either way, this plot is used to contrast the leader's leadership style against what the lancer's would be. A frequent ending for this plot is for The Lancer to gladly give up the reins of power while The Leader often notes that the team will be in excellent hands the next time he is absent.
This trope is named for the man-at-arms of The Middle Ages, the term for a professional soldier. While the term also encompasses the members of the knightly class, a man-at-arms was not necessarily a knight. They were also men of lesser financial and social status than knights, but were equally trained and equipped to fight on horseback in full armour and with sword and lance, just like their social superiors. In this regard, he is most recognizable as King Arthur's right-hand man, Kay, who was the greatest warrior of Arthur in source Welsh legends (and who was later demoted to plucky comic relief) - either way, as a constant Foil for our noble hero, he fits this trope to the letter.