Alexander Lightwood is the third and youngest child of Gabriel and Cecily Lightwood.
Born in 1900, Alexander is the younger sibling of Anna and Christopher, born 13 years after the latter. Like Christopher, he inherited their mother's blue eyes.
- According to a found family tree, Alexander died in 1971 at the age of 70 or 71.
- Alec Lightwood, one of his descendants, shares the same name as him.