Jupiter

Jupiter is the Roman form of Zeus, Lord of the Sky, King of the gods, and the god of lightning. As Jupiter, he becomes more disciplined, militaristic, and warlike. Camp Jupiter was named after him. The Greeks envisioned Zeus as a powerful and proud being while the Romans envisioned Jupiter as a more formal and stoic version of his Greek counterpart. He was mentioned in the series many times.

The Lost Hero
When Jason is found by Annabeth at the Grand Canyon, he calls the gods by their Roman names instead of the Greek ones. Later that day, Chiron who knows a small bit about his past says that Jason was already claimed. To prove this, Jason calls down a bolt of lightning to prove he is a son of Zeus, but in actuality he is a son of Jupiter.

Later when meeting with Thalia, she talks about how Zeus returned to their mother once after she was born. She described him as being different from how she remembered him, being more forbearing, mature, and severe. This is because Ms. Grace had gained the attention of not Zeus, but Jupiter.