From Fire Emblem Wiki
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 instead. In the Tellius series it became an offensive affinity.
[edit] 
Dark Affinity Bonuses in Fire Emblem: Fūin no Tsurugi
| Support Level | Attack | Hit | Defense | Evasion | Critical
|
| C
| + 0.0% | + 2.5% | + 0.0% | + 2.5% | + 2.5%
|
| B
| + 0.0% | + 5.0% | + 0.0% | + 2.5% | + 5.0%
|
| A
| + 0.0% | + 7.5% | + 0.0% | + 2.5% | + 7.5%
|
[edit] Characters with the Dark Affinity in Fire Emblem: Fūin no Tsurugi
Astol, Merlinus, Percival, Rutger, Sophia, Zealot
[edit] Dark Affinity Bonuses in Fire Emblem: Rekka no Ken
| Support Level | Attack | Hit | Defense | Evasion | Critical
|
| C
| + 0.0% | + 2.5% | + 0.0% | + 2.5% | + 2.5%
|
| B
| + 0.0% | + 5.0% | + 0.0% | + 2.5% | + 5.0%
|
| A
| + 0.0% | + 7.5% | + 0.0% | + 2.5% | + 7.5%
|
[edit] Characters with the Dark Affinity in Fire Emblem: Rekka no Ken
Rath, Isadora, Karla, Merlinus
[edit] Dark Affinity Bonuses in Fire Emblem: The Sacred Stones
| Support Level | Attack | Hit | Defense | Evasion | Critical
|
| C
| + 0.0% | + 2.5% | + 0.0% | + 2.5% | + 2.5%
|
| B
| + 0.0% | + 5.0% | + 0.0% | + 2.5% | + 5.0%
|
| A
| + 0.0% | + 7.5% | + 0.0% | + 2.5% | + 7.5%
|
[edit] Characters with the Dark Affinity in Fire Emblem: The Sacred Stones
Knoll, Orson, Rennac, Riev
[edit] 
Dark Affinity Bonuses in Fire Emblem: Path of Radiance
| Support Level | Attack | Hit | Defense | Evasion
|
| C
| + 0.5% | + 0.0% | + 0.0% | + 2.5%
|
| B
| + 1.0% | + 0.0% | + 0.0% | + 5.0%
|
| A
| + 1.5% | + 0.0% | + 0.0% | + 7.5%
|
[edit] Characters with the Dark Affinity in Fire Emblem: Path of Radiance
Calill, Giffca, Soren, Naesala
[edit] 
Dark Affinity Bonuses in Fire Emblem: Radiant Dawn
| Support Level | Attack | Hit | Defense | Evasion
|
| C
| + 0.5% | + 0.0% | + 0.0% | + 2.5%
|
| B
| + 1.0% | + 0.0% | + 0.0% | + 5.0%
|
| A
| + 1.5% | + 0.0% | + 0.0% | + 7.5%
|
[edit] Characters with the Dark Affinity in Fire Emblem: Radiant Dawn
Black Knight, Calill, Giffca, Micaiah, Naesala, Soren
