Alexis "Lexi" Denise Glass-Mason is Tom Mason's and Anne Glass's daughter.
At least 9 months into the invasion Anne found out she was pregnant with Tom's child.(A More Perfect Union) The baby was born at least 7 months later. Tom announced the baby's name at the forum. Alexis showed signs of being "advanced" early on, being able to pull herself up in her bed, move around freely, and talk only a week after being born- actions which were only seen by Anne and which frightened her.(On Thin Ice) Sometime later, Anne has Roger Kadar run a blood test on Alexis and found she had alien DNA in her blood stream.(At All Costs) Dr. Kadar later explained to Weaver and others that this meant she was a human-alien hybrid, an entirely new species. Fearing that the revelation of a hybrid child might cause the citizens of Charleston to harm her child, Anne knocked out both Kadar and her friend Lourdes (who was secretly implanted with Espheni Eye worms, making her an unwilling Espheni agent) and fled, but she and Alexis were soon captured by the Espheni. (Search and Recover)
2 months after Alexis was born, Alexis and Anne are found by Tom after the attack in Boston. Alexis' alien DNA has caused rapid growth; she is physically now six years old and fully capable of speech. When they got to safety in the resistance, Anne spent her time resting in a tent. Meanwhile, we see Alexis approaching the cage for Lourdes who's still bound to the eye worms. As Alexis touches her chin, she cures Lourdes and the eye worms are dragged to her hand instead. As her father Tom and Weaver are watching, she kills the eye worms by closing both of her hands and later when she opens them, the eye worms have vanished and turned into dust.(Brazil)
She marks the beginning of an entirely new species. Unlike any other human, Alexis has alien DNA in her blood.