Speed is a stat that usually affects the number of strikes a character can make. It can also affect the evasion ability of a character.
It is known that, at least since Sword of Seals, that Speedwings can be used to raise the speed of a character by two points.
Mechanics of Speed
This section is split under two sub-sections: GBA and 3-D Fire Emblem games.
GBA games
Attack speed is calculated as follows:
Character's speed minus (Weapon Weight minus User's Con - only applies if this result is greater than zero)
Evasion is affected by Speed X 2 + Luck - (Weapon Weight - User's Con, only if greater than zero)X 2 + support bonus
3-D games
Attack speed is calculated as follows:
Character's speed minus (Weapon Weight minus User's Strength - only applies if this result is greater than zero)
Evasion is affected by Speed X 2 + Luck - (Weapon Weight - User's Strength, only if greater than zero)X 2 + support bonus