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 earth affinity in each game.
The Earth affinity is considered one of the more powerful affinities thanks to the high evasion bonus it gives. Characters with this affinity are often preferred supporting one another for maximum evasion at the A-level support.
[edit] 
Earth Affinity Bonuses in Fire Emblem: Path of Radiance
| Support Level | Attack | Hit | Defense | Evasion
|
| C
| + 0.0% | + 0.0% | + 0.0% | + 5.0%
|
| B
| + 0.0% | + 0.0% | + 0.0% | + 10.0%
|
| A
| + 0.0% | + 0.0% | + 0.0% | + 15.0%
|
[edit] List of Characters with the Earth Affinity in Fire Emblem: Path of Radiance
Ike, Lucia, Oscar, Tanith, Zihark
[edit] 
Earth Affinity Bonuses in Fire Emblem: Radiant Dawn
| Support Level | Attack | Hit | Defense | Evasion
|
| C
| + 0.0% | + 0.0% | + 0.0% | + 5.0%
|
| B
| + 0.0% | + 0.0% | + 0.0% | + 10.0%
|
| A
| + 0.0% | + 0.0% | + 0.0% | + 15.0%
|
[edit] List of characters with Earth Affinity in Fire Emblem: Radiant Dawn
Volug, Nolan, Zihark, Fiona, Lucia, Ike, Oscar, Ena, Renning
