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 light affinity in each game.
The Light affinity grants offensive and defensive bonuses similar to the Anima affinity, though it gives secondary boosts to Hit and Critical hit rate instead of Evasion and Critical Evade. Because of this, Light is more offensive where Anima is more defensive. Along with the Fire affinity, it does not give Critical Evade by itself. In the Tellius series, it became a defensive affinity. It is a relatively common affinity for both characters and maps.