You will meet Lucia very early on in Colorado Springs, as part of the "Cornered Rats" quest. The Patriarch sends you to Sheriff Daisy at the Marshal headquarters about Dorseys still in the Garden of the Gods. Lucia is there as well, and is part of the conversations for the quest, so offers to join you.
She has a very good pistol and does a lot of damage early on, but has a very low strength, so is quite squishy. Armor her up and raise her strength and she's a good squad member.