In the original Fire Emblem: Ankoku Ryu to Hikari no Tsurugi, a pair of playable cavalier units are introduced early on named Cain and Abel. Cain is identifiable by his red hair and armor, while Abel sports green hair and armor. However, while they are named for figures in the Bible, they are not brothers, nor does their relationship otherwise mirror the biblical Cain and Abel. Both are knights in the service of Prince Marth of Altea.
In later games, similar pairs of red and green cavalier units appear. Like Cain and Abel, these cavaliers are typically closely associated with one another and share some manner of friendship or rivalry. In most games, these units share similar personality traits, with the red "Cain" cavalier a more outgoing or excitable figure and the green "Abel" more subdued. These units also typically favor certain stats for growth potential, with Cain units favoring Skill and Speed and the Abels Hit Points, Strength, and Luck. (Exceptions to this rule exist in both Seisen no Keifu and Fire Emblem, in which these traits are flipped between the games' respective Cains and Abels.)
Cains and Abels by Game