what motivates the main character

Lisa Nelson is a ten-year-old girl who is left to fend for herself and care for her brother when her parents, and all adults, are killed off by a virus. Lisa spends her time thinking, planning, and solving problems. When she loses Glenbard to Tom Logan's attacks, she blames herself for not having the foresight to prevent Glenbard being taken from her. She feels she earned Glenbard and must continue to earn it by thinking up the ideas to protect it.