Technically speaking, Lucius Verus II is the son of Lucilla and her deceased husband, Lucius Verus. The boy’s mother is a very real historical figure and was the daughter of an emperor (Marcus Aurelius), the sister of an emperor (Commodus), and a Roman Empress herself.
Lucilla filled that last role as the wife of Lucius Verus. According to Roman history, Verus Sr. co-ruled with Lucilla’s father, Marcus Aurelius, for a while. While Verus died before “Gladiator” begins, the royal couple reportedly had several children together. Some sources refer to a boy named Lucius Verus, but tragically, none of the children seem to have lived very long.
That means the fact that Lucius is still alive in “Gladiator” is already bending historical truth. To reveal him as the bastard son of a fictional gladiator will certainly cross the line into historical fiction — but it’s not like Ridley Scott is afraid to play in that sandbox, anyway. His historical dramas are loaded with historical inaccuracies, most of which admittedly make the story better.
