Demigods are people that have one godly parent and one mortal parent. Demigods can be Roman or Greek. Very rarely does a Greek god marry someone and has a kid then comes back as a Roman god and has another kid. So this means that rarely does a Greek demigod have a Roman demigod as a sibling. Percy Jackson, or Perseus, the slayer of Medusa, is a demigod. The hero Achilles is also a demigod. Hercules, the slayer of the Hydra, the capturer of Cerberus, is a demigod. Demigods were needed by the gods when the giants attacked Mount Olympus.