Julian Solo is the heir to the Solo family, a immensely wealthy and powerful clan that made its fortune through global maritime trade and shipping. As a sixteen-year-old, he has already assumed leadership of the family's vast business empire following the recent death of his parents, a position that has shaped his worldview and ambitions. He is known for his strikingly handsome appearance, characterized by long blonde or blue hair and elegant bearing, which is noted even by those who have not met him personally.

As a young man, Julian possesses a complex personality. He is charismatic, idealistic, and naturally charming, yet also ambitious and accustomed to having his desires met without resistance. His ambition is particularly focused on the sea, which his family has long controlled. He believes that whoever commands the oceans can command the world, and he intends to expand his family's business globally to achieve this dominance. While he has a genuine, deep-rooted love for the sea and its creatures stemming from his childhood, his perspective has matured to see the ocean as both a wonder and a source of immense power. This ambition and sense of entitlement are most clearly demonstrated when he meets Saori Kido at his sixteenth birthday party. Struck by what he feels is a fated connection with her, he proposes marriage on the spot, believing his offer is one that no one could refuse. Her rejection stuns him, wounding his pride and fundamentally challenging his worldview.

This bruised pride leads him to follow a mysterious light to the ruins of a temple of Poseidon at Cape Sounion. There, he finds the god's trident and is met by Thetis the Mermaid, a servant of Poseidon's army. Thetis reveals that Julian is the chosen host for the reincarnation of the sea god Poseidon, a destiny for which the Solo family has been groomed since mythological times. She then takes him to the underwater Temple of Poseidon. Once there, Julian dons the Scales of Poseidon, and the god's spirit begins to take control of him, though his own feelings and memories initially remain present.

In his role as the host of Poseidon, Julian becomes a primary antagonist. The god's will, which gradually overcomes Julian's own personality, aims to punish corrupt humanity by causing great floods and earthquakes to purify the Earth and start anew. As Poseidon, he orchestrates the flooding of the world, has Saori Kido kidnapped, and imprisons her within the Main Breadwinner, a massive pillar that sustains his underwater kingdom, to both eliminate a rival and strengthen his domain. He offers Saori a chance to rule beside him in his new utopia, but her repeated refusal solidifies their conflict. During the ensuing battle with the Bronze Saints, a powerful attack momentarily awakens Julian's own consciousness, causing him to briefly wonder where he is and what is happening before Poseidon's soul fully suppresses him for the rest of the fight.

Key relationships define much of Julian's human life. His connection to Saori Kido is central; it is his personal interest in her that inadvertently triggers his awakening as Poseidon, and the goddess Athena becomes his divine adversary. Another significant relationship is with Sorrento, the Mariner General of Siren. Unlike other Mariners, Sorrento serves Poseidon out of free will and maintains a bond with Julian as a person. Following his defeat, it is Sorrento who accompanies Julian on his journey of atonement, and Sorrento continues to show respect and loyalty to his master even when he is not possessed by the god. The Mermaid Thetis also plays a crucial role, serving as his guide to his destiny as Poseidon's host.

After Poseidon is defeated and sealed once more, Julian Solo returns to his human life with no memory of the events that transpired during his possession. However, the experience leaves a profound mark on him. Demonstrating an altruistic side that was always present beneath his ego, he donates a great portion of his inherited fortune to aid victims of the floods caused by his divine alter ego. He then embarks on a journey of self-imposed redemption, traveling the world with Sorrento to help orphaned children. Although he returns to a normal life, his role as Poseidon's host is not yet over. During the battle against Hades, Poseidon temporarily possesses Julian's body again to send the five Gold Cloths to the Bronze Saints fighting in Elysium, a crucial act of aid from an unexpected source. After the crisis passes, Julian smiles, satisfied, with no memory of this divine intervention.

As the earthly vessel for an Olympian god, Julian's notable abilities are not his own but those of Poseidon manifesting through him. In this state, he possesses immense cosmic power, described as being so vast it nearly covers the universe. With his divine trident, he can unleash devastating attacks and control natural forces like storms and earthquakes. One of his primary defensive abilities is an automatic reflection of any attack launched against him, repelling the damage back to the assailant. Only a concentrated attack from multiple powerful Saints, such as Seiya using the Sagittarius Gold Cloth's arrow with the combined cosmos of his friends, can break through this divine defense and cause him injury. Even then, such an attack only serves to awaken Poseidon's full power. As a god, his sheer divine aura and will are enough to defeat warriors like Shaina without him even moving his body.