Loosely, affinity determines a character's stat boosts that are gained from supports. Supports depend on the affinities of the two characters that are being bonded, and they determine the type of bonus given. The following tables are a compilation of bonuses given by the dark affinity in each game.
The Dark affinity is a rarity in the Fire Emblem series, as only a few characters possess it in each game. It gives neither offensive nor defensive bonuses in the GBA games, making it somewhat of an oddity, as it gives full Hit, Evade, and Critical hit rate instead. In the Tellius series it became an offensive affinity. It is very desirable affinity for Berserkers as it helps with their critical hit rate and helps them connect blows and reduce the chance they get hit critically as the class's most pronounced features are it's low Skill, low Luck, and high Critical hit rate, high offensive potential, among others.