Terra is not a normal human being, but instead is a unique idividual due to her parentage and where she actually comes from.
Her mother, Madeline, was one who hated the human world and tried to leave the world causing her to unintentionally find herself in the world of espers. She met Maduin, Terra's father, when he found out about Madeline and became curious about the humans that lived on the other side.
Over time, as Maduin stayed by Madeline's side, he became interested in Madeline, and the esper elder takes notice of this and tells Maduine that Madeline cannot stay in their world. The day after the elder tells Maduin the information, Maduin finds out that Madeline had aleady left and he follows after her. He tries to convince her to stay and she eventually does decide to stay.
Over time, Madeline and Maduin become closer and soon end up as lover. Before time is through, Madeline is pregnant with Terra and she is born into the esper world.
