Cassius is a Gold of the Martian house Bellona, the son of their leader Tiberius. He's shown to be arrogant and careless, but values the people he keeps close. He is the favourite of his family; his twin brother, Julian, was killed in the Passage , while his older brother, Karnus, is portrayed as the 'monster' that cleans up family business.
Cassius was born to one of the most powerful and wealthiest families on Mars. He had a twin brother Julian and they were the youngest of the children of Tiberius and Julia. Cassius is also the Tiberius' favourite son. Cassius loved his twin brother Julian very dearly and always looked after him.
Naturally talented and clever he was good at everything he did, but because of his good looks and Gold origin, he became more arrogant and vain after his time at the Institute.
Cassius is an arrogant gold, who at times couldn't care less about his actions. He makes friends with Darrow, but has different sides to him at different events in the story. He cares for Darrow at the beginning, and they are good friends, but when he finds out that he killed Julian, he is angered and has an urge to kill Darrow.
Cassius au Bellona is described as a perfect Gold. He has golden curly hair and eyes. Cassius is very handsome and has a strong body due to his training.
Darrow first described him as having a "wide and white" smile, with "smooth and lustrous" skin. 
"His hair is thick and lustrous. His chin cleft, hands steady. Shoulders powerful, made for war. Face made for the hearts of court" ~Golden Son, (page 98)
Cassius was placed in the same House as Darrow where they became close friends with Darrow, until he learns it was him that killed his brother, Julian. Cassius took Darrow to a secluded area and tried to kill him in the mud.
After attempting to stop Darrow in the Arctic, Cassius gets captured by Darrow, Mustang, and Holiday. He spends four months captive in the Morning Star. Later, Cassius was allowed to escape before the ship got to Earth, but he uses this moment of weakness to "kill" Sevro and hold hostage Darrow and Mustang. He plans on delivering the two and Sevro's body to the Sovereign. But this was all part of another plan. Cassius aids in the killing of the Sovereign, "because she lied."
Cassius and Darrow have a complicated relationship, with them initially befriending one another. They fought together at the Institute and worked well side by side. However, Cassius learns that Darrow killed Julian, Cassius's twin brother, during the Passage, and thus betrays Darrow and stabs him. Darrow is able to survive and even achieves victory at the Institute. Cassius, defeated, swears revenge against Darrow for the death of his brother.
In Golden Son, Cassius attempts to use his new relationship with Virginia to hurt Darrow, but he is soon after beaten badly by him and publicly humiliated in a duel. Cassius loses his hand in the duel and loses face before the Sovereign. His family is later entirely massacred, and Darrow is blamed for it. Cassius is eventually one of the key reasons for Darrow's downfall and capture in Golden Son, killing Ares and taking Darrow's ring from him.
In Morning Star, they are reunited after a year, where Darrow has been badly tortured. Cassius is shocked by how Darrow has been treated, and even shows him some pity by giving him a cloak to warm him. Darrow escapes from captivity, and soon after defeats Cassius and imprisons him instead. Darrow treats Cassius well, however, and slowly they are able to reconcile their old friendship after Roque's death and funeral. Cassius learns the truth that the Sovereign was behind the death of his family, and aids Darrow, Mustang, and Sevro in defeating her. Cassius leaves shortly after with Lysander in a self-imposed exile, though not before returning Darrow's ring and parting with him in forgiveness and friendship.
Cassius meets Virginia at the Institute, knowing her as Mustang and not knowing that she is the daughter of Nero au Augustus, an arch rival to his family. He finds her beautiful and intimidating, and somewhat like his mother, while Mustang finds Cassius to be foolish but handsome. However, they have little to do with one another, as Mustang later helps Cassius's enemy Darrow achieve victory in the Institute.
In Golden Son, Cassius and Virginia are in a relationship together in an effort to make peace between their families. They are practically engaged, and Cassius attempts to use his new relationship to hurt Darrow, who has feelings for Virginia. After Cassius's defeat to Darrow and near death, it is Virginia who convinces Darrow to spare his life and tends to his wounds after. However, Virginia ends her relationship with Cassius in order to join Darrow and her father, and becomes Cassius's enemy. She begins a relationship with Darrow and reveals that she had only been with Cassius to ensure the safety and security of her family, and not out of love or deep romantic feelings.
In Morning Star, they interact very little, though Virginia again convinces Darrow to spare him. They work together later to defeat the Sovereign, and Cassius leaves in self-exile as Virginia stays with Darrow and their son together to reshape the Society.
"I am Cassius au Bellona, son of Tiberius, son of Julia, Morning Knight, and my honor remains." -- Cassius in Iron Gold
"We all deserve the worms, Raa." -- Cassius to Seraphina in Iron Gold 
- Gaius Cassius Longinus was one of the people that plotted to kill Julius Caesar.