WARNING: The following contains spoilers for Episode 9 of The Strongest Sage With The Weakest Crest, "The Strongest Sage Duels," now streaming on Crunchyroll.
Gaius was a powerful sage who existed 5,000 years prior to the current timeline of The Strongest Sage With The Weakest Crest. In his day, Gaius was greatly feared by demons due to his superior combat skills and his ability to make powerful weapons against them. Despite having built a strong reputation as a powerful sorcerer and a formidable foe against demons, Gaius still felt his skills and power levels weren't good enough. Having been born with only one crest, Gaius felt limited by his natural-born powers and couldn't realize his full potential. As such, he desired to possess the more powerful four crests but could only obtain them by being reborn. To that end, he used a magic spell to induce his reincarnation.
In his current incarnation, Gaius is a 12-year-old Second Academy student named Matthias "Matty" Hildesheimer residing in the kingdom of Eis. At the Second Academy, Matty quickly climbed the ranks as the most powerful sorcerer and even helped elevate the school's status by teaching the students silent incantation. He also befriended classmates Lurie and Alma and reunited with Iris the Darkness Dragon, an old friend of his from his days as Gaius. He is even favored by his king, as he was a staple in setting up a powerful magic barrier over the kingdom of Eis to repel demonic invasions. Having completed this task, Matty, accompanied by his friends, decided to continue following the demons' trail along the Dragon Vein.
Since departing the school, Matty paid a visit to the town of Melkia which was built atop a large dungeon. Like the kingdom of Eis, the small town had been targeted by demons and was even infiltrated under the guidance of a mysterious ancient demon who happens to know Gaius. While the ancient demon suspects Matty might be Gaius' reincarnation, he also has doubts due to Matty not being quick to kill his subordinates. He challenged Matty to locate him, but Matty was only able to pick up a false remote location. Unfortunately for the ancient demon, Matty recorded his magic signature onto a magistone and is determined to find him. However, to do that, he needs to reclaim a device he created as Gaius outside the kingdom's borders.
In Episode 9 of The Strongest Sage, Matty returns with his friends at the Second Academy to get permission from the king to cross the border to the Radinia Alliance, a group of allied kingdoms. Within the borders of the Radinia Alliance is a place known as the Folkia Region, which is where Gaius' device is located. While the king's pass allows Matty and friends to enter the Radinia Alliance, to access the Folkia region, they need to pass the allied kingdoms' Rank A exam, which is comprised of a mock battle between an applicant and an appointed proctor. The proctor who was originally assigned to Matty and friends passes on testing them due to Matty's reputation as a powerful demon slayer. In his place, a new proctor in the form of Giluas challenges Matty to a duel.
In contrast with Matty's previous opponents, Giluas is an S-ranking adventurer, meaning he is a high-ranking combatant. As such, he is highly trained in battle tactics and power-enhancement magic, although he is unaware of the latter as he instinctively uses it without thought. He also has a reputation of never losing a fight, just like Matty. During his duel with Matty, both combatants study each other, and Matty for once admits he is paired off with a fighter who matches him in skill. The one skill that impresses Matty the most is Giluas' ability to study his opponent's movements and use them against that opponent.
At one point, Giluas uses Matty's tactics against him, although he is not easily defeated. Matty quickly shows Giluas that he too had been studying his moves, but rather than use his own battle tactics against him, Matty takes advantage of his arrogance. This facilitates a win for Matty, and while Giluas hates the fact that he lost against a 12-year-old boy, he also finds himself admiring Matty for his level of skill. He then asks Matty to teach him his skills, and Matty supplies him with a copy of a textbook he himself wrote for perfecting magic use. Since Matty is equally impressed by Giluas, he tells him he already uses magic in battle and that reading the book will teach him to better control it.
Giluas invites Matty to take the exam for Rank S, but Matty declines since he only needs Rank A to get to the Folkia region -- a place with which Gliuas is actually familiar. Having befriended Matty by this point, Giluas warns Matty that people go missing when they reach Folkia, indicating a demonic invasion. He therefore agrees to help Matty in combat when summoned.
