Lucius, also known as the Ancient Adventurer, is a historical figure from approximately one hundred years before the events of Pokémon Horizons: The Series. He was a Pokémon Trainer and explorer who, after hearing fairy tales about a mysterious paradise called Laqua, set out on a journey to find it. He was accompanied by a shiny black Rayquaza and formed a bond with six other Pokémon that became known as the Six Heroes: Arboliva, a Galarian Moltres, Lapras, Kleavor, Entei, and the shiny Rayquaza. Additionally, the Legendary Pokémon Terapagos traveled with him, often taking the form of a pendant. Lucius was one of the three founders of the organization known as the Explorers, alongside Gibeon and Rystal.

In terms of personality, accounts describe Lucius as cocky and reckless, often challenging wild Pokémon even when outmatched. His motivations were driven by a desire for adventure and discovery, particularly the search for Laqua, which he believed to be a real place where Legendary Pokémon lived in peace. This quest ultimately led him and his partners to reach Laqua, though the exact events that followed remain shrouded in mystery. His exploits became legendary, and his story was passed down through generations, eventually inspiring both Liko and Roy in the present day.

Lucius's role in the story is primarily as a legendary figure whose past drives the main narrative. Liko and Roy, along with the Rising Volt Tacklers, seek to reunite the Six Heroes and return Terapagos to Laqua, fulfilling what they believe to be Lucius's unfinished mission. Lucius's legacy is deeply tied to Liko's family: he is revealed to be her great-great-grandfather, and his wife Rystal is Liko's great-great-grandmother. Liko inherits a pendant that contains Terapagos, and her blue hair shares a color with Lucius's own blue hair, hinting at their blood connection. His old diary and a mysterious belt also provide clues that guide the protagonists. Through these artifacts and the memories of his Pokémon, Lucius's influence continues to shape the journey of Liko, Roy, and their friends.

Notable abilities include his skill as a Trainer capable of commanding a team that includes multiple Legendary and rare Pokémon, such as a shiny Rayquaza and Terapagos. His bond with the Six Heroes allowed him to travel across regions and ultimately reach the mythical Laqua. Though he lived long ago, his courage and determination left a lasting mark on the world, making him a central figure in the search for the paradise and the mysteries of Terapagos.