Alejandra was a historian and a tinkerer. She implies that she a sex slave for 4 years to a master who cared enough to educate her in literacy, math, history and to some degree, science. After her master got married, she was sold to Hannibal Hamlin, who promptly freed her.
If the Lone Wanderer asks her what she does, she will admit that she wants to build a shrine to Abraham Lincoln.
It is unknown if this has anything to do with the strange shrine that already exists to him inside the basement of the Arlington House, but it is very improbable, as they aren't really linked.
She passes on her knowledge to the other freed slaves who wish to learn and hopes to create a school for freed slaves in the Washington Memorial.
↑Fallout 3 Official Game Guide, page 43: "There is a band of abolitionist slaves deep in the Wasteland. They are all escaped slaves, seeking to end slavery for all time. You can learn more about them in Miscellaneous Quest: Head of State."