Constitution

From Fire Emblem Wiki

Constitution (also known as Build and is shown as Con in game) in Fire Emblem is a representation of how big a character/unit is. Constitution is also affected by characters that usually have Mounts (Pegasus Knights, Cavaliers, etc). Constitution determines which weapons units can carry without suffering a penalty to their attack speed in battle, as well as which other units they can rescue and capture.

Similar to Movement, this stat only increases with the use of an item or through class change.

The item Body Ring can increase the Con stat by 2, just as the Boots and the Talisman increase Movement and Resistance by 2 respectivly.