The speed of a character or unit can depend on the personal statistics of the character or unit and the class of the unit. For example, a Knight is going to have naturally slower speed than a Myrmidon, so a Myrmidon is naturally more likely to double attack and evade a counterattack in a fight against the Knight. However, the concept of Constitution makes this slightly more complex; if the weight of a weapon is higher than the unit's Con, the unit will suffer from a speed penalty. Weight-Con = Speed Penalty. For example, if a Pegasus Knight with a Con of 5 uses an Iron Lance ( Wt. 8) to attack an enemy she will suffer from a penalty of 3 Speed. What is left is her Battle Speed. If her Battle Speed is 4 higher than the enemy's, she will double him. This is raised to requiring 5 higher in later titles.
A Speed Ring or a Speedwing can be used to raise the speed of a character. Of course, the number of speed-boosting items present in each game are limited, so often they are used to boost the speed of a character with low speed, helping their evasion and their chances of double-attacking. Brave Weapons are also affected, if their speed stat is 5+ from their opponent, they shall attack 4 times instead of 2.
In modern Fire Emblem games, such as Fire Emblem: Path of Radiance or Fire Emblem: Radiant Dawn, the Strength Stat is used in place of Constitution to determine the penalty, and Fire Emblem Awakening discards the Speed Penalty entirely. Fire Emblem Fates, while also disregarding weight and con impliments a new feature. Weapons like steel swords/axes/lances will come with a penalty of -3 effective speed. Throwing weapons like the Javelin or Hand Axe come with a penalty of -5 effective speed and cannot double. The Raider series of weapons grant +3 effective speed.