The Captain has a hand in just about every job for a group. Alone a Captain is a capable warrior and healer, but in a group, they improve a fellowship to an exceptional level. Though they aren’t the best in any one area (even when they are traited that way), they can fill just about any role in a group to a limited extent. Equipped with a Herald and plenty of offensive and defensive capabilities, the Captain is a class that requires some thought and focus to mastering.
From the Lorebook:
Captains are armoured melee fighters and masters of buffing; skills that enhance allies. They may summon a Herald companion to fight by their side, ensuring they always have an ally to buff. They can also Mark foes, giving benefits to all who attack them. In fellowships, a Captain supports his allies with healing and buffs, while still dealing respectable damage.
The captain is a true hybrid. They would possibly be closest to a paladin if they were in another game. They wear heavy armor and fight with big weapons but also buff, protect and are good backup healers.