Preston Garvey is the "last minutemen" that you encounter in the Museum of Freedom earlier along with a handful of survivors. Completing his quest "The First Step" will allow him to selectable as a companion.
Extra skill/ability
When visiting Minutemen-controlled settlements, the settlers may talk to him and give him various items, weapons, or ammo which you can get from his inventory.
Unique Perk: United We Stand
Damage dealt +20% and damage reduction +20% when facing three or more enemies
Approval
Helping settlers and other normal citizens
Offering help
Customizing weapons
Disapproval
Stealing
Brotherhood of Steel
Asking monetary rewards
Attacking another minuteman
Getting addicted to chems
Giving chems to Mama Murphy
Using the Fat Man