Grace Armstrong (Grace Under Fire)
Find Grace in the Lamb of God Church, well west of Fall's End in John's territory.
Arrive to find her in the belltower, sniping cultists left and right.
Give her a hand, then join her by taking one of the blue exterior ladders up (there don't seem to be any interior stairs).
Speak with Grace to begin her loyalty mission: Grace Under Fire.
You may also want to snag the map on the small table behind her.
Your goal here to defend the three marked mausoleums against several waves of cultists. The mausoleums are actually pretty tough, so take your time lining up shots- better to spend an extra second on a headshot, then spend five lining up the next one.
Move around the roof to get a better angle on attackers and (optionally), keep hidden, so you can knock down hostiles while they focus on Grace. You can even use a similar strategy at ground level, if you're more a hands-on type.
Either way, the toughest enemies you'll face here will be the flame throwers. They can soak a lot of damage (though are vulnerable to takedowns), and they do have enough range to hit the roof.
The winning strategy is to take aim at their fuel tanks, which will not only kill the flamer, but anyone nearby (note: don't be nearby).
When you've killed the last of them, talk to Grace again to unlock her as a companion, and complete the mission.
Note that, across the road from the church (east), is a turret-gun and an unattended rocket launcher. If you don't have one yet, this is an easy way to add it to your arsenal.