The titular rider (and by extension, Gills and Another Agito) is one of four primary Kamen Riders in the entire series (and the only Heisei Rider) whose powers are not directly related to the monsters fought in this seriesnote The other three riders being Kamen Rider X, whose powers were originally intended for marine exploration; Kamen Rider Super-1, whose powers were originally meant for space exploration, and Kamen Rider J, who was created by benevolent earth spirits - while the Lords were created by the Overlord of Darkness in response to his brother, the Overlord of Light, giving humanity the Seeds of Agito so that humans could become more powerful than his brother, the Lords themselves do not use the Seeds of Agito as the source of their power. This makes Agito an abnormality on the Cross of Fire.
Follow the Leader: Following up the lone Kuuga with the multi-Rider Agito proved a success, resulting in Ryuki having even more Riders with multi-Rider shows being the norm beyond.
Milestone Celebration: This series was made during Kamen Rider's 30th anniversary and contains homages to past Kamen Rider Series. Some of the monsters are based on old villains, and you can make the case the lead Riders themselves are based on previous Kamen Riders. It also homages the Metal Heroes series which Kamen Rider took the place of and incorporated some concepts.
The same lower face mask is used by most of the Lords. Other parts like the gloves and chestpieces can occasionally be seen repainted and reused, especially on the mammal-based Lords.
Real Life Writes the Plot: The reason Gills hasn't been seen much since Agito endednote Kamen Rider Decade used Exceed Gills instead, and his brief cameo in the 40th anniversary movie was heavily in shadow is because the opening "jaw" on the helmet broke from overuse and Toei is apparently unwilling to repair a costume for a character whose story is already over.
For years there were rumors that the Gills and Another Agito suits were destroyed by water damage due to a leak in the Toei warehouse, but Kino's appearance in Kamen Rider Ex Aid Kamen Sentai Gorider seems to have put that rumor to rest.
Star-Making Role: The is Seiji Takaiwa's first stint as a Suit Actor for Kamen Rider. Until he relinquished the role in Zero-One, he's played EVERY single primary Rider except for Hibiki, with over 18 Heisei Riders under his belt (or rather their belts).
Shinichiro Shirakura's proposal for the 2001 Kamen Rider was humans using cards to form contracts with monsters, which would be realized for the next year with Ryuki.
There was an idea for the series to feature 10 Kamen Riders, which would be reused in Ryuki (although with the number of Riders increased to thirteen.
As noted on the main page, Agito was originally intended to be a sequel to Kamen Rider Kuuga, but the writers of the latter series objected, stating that doing so would have made Godai's battles meaningless.
Agito was actually originally planned on being the final Kamen Rider series. Then the 9/11 terrorist attacks in the United States occured, and it was decided that kids needed a show to teach them justice, so these plans were scrappednote reportedly, Ishinomori was planning it the same time he was planning Kuuga, so it's likely that the original plan was to finish what he had started.
At Japan Worlds Heroes 2017, Noboru Kaneko, who would later play GaoRed, mentioned that he auditioned for both the title role and Makoto Hikawa.
Yusuke Kosaka, the son of X-Rider lead Ryo Hayami and his co-star Naoko Miyama, was famously in the final selection in the auditions for Agito only to be rejected when his connection to an earlier Rider actor was found out.
The original concept for G3-X would've had him wielding a riot shield, which ultimately didn't become part of his arsenal.
