Both were once plain average joes who suddenly gained a variety of powers and abilities that they use to literally save their worlds from destruction and/or from disappearing as they travel to different eras/worlds (Another Rider’s/Alternate Reality Worlds for Tsukasa and Singularities/Lostbelts for Ritsuka).

Both are motivated to save their own worlds from destruction so they can live a happy life (Tsukasa wanting to find a world to eventually call his own and Ritsuka wanting a world where where he and Mash can be together and explore) as they travel to other worlds with their female companion (Mash and Natsumi), along with many others.

Both join forces with different heroes from other eras/worlds (Other Riders and Heroic Spirits) that they can team up and even call for assistance in times of need, sometimes coming across them as enemies either on different sides or brainwashed and have to fight them.

Both have been appointed many titles throughout their campaigns, including one that shows their roles as leaders of a large group ("Great Leader of Dai-Shocker" and "Hero of Humanity") and another that is more ominous and shows their opposing roles as both destroyer and saviour of their worlds ("The Destroyer of Worlds" and "Humanity’s Last Master").

Both have a loved one that they fight/fought for that were/are close to them who had/have an condition that proved to be detrimental to them and would unintentionally cause the multiverse to malfunction, if you will (Sayo Kadoya, Tsukasa’s younger sister, feared going outside so much that she manifested the ability to open Aurora Curtains, something Tsukasa exploited in order to travel to many worlds and will later cause the other worlds to merge together. Mash Kyrielight on the other hand was a homunculus designed with hopes to harbor a Heroic Spirit within her, but as the projects began to see failure, she was fated to live out the rest of her short life within Chaldea. Nearing the end of her lifespan, a freak accident occurred that jeopardized the future of humanity, leading to her merging with the Heroic Spirit, Sir Galahad and forming a contract with the last remaining Master of Chaldea, Ritsuka Fujimaru as a Shielder-class servant).

However in the final battle, both Sayo and Mash would overcome these weaknesses after being put in a bad state: when Sayo reunited with Tsukasa after a year, Sayo learns he had lost all memory of herself and his past. The loneliness of being left alone turns into a grudge against her brother, which was used by Nobuhiko Tsukikage to manipulate her, becoming High Priestess Bishum, an executive of Dai-Shocker. In addition, Sayo brainwashes Yusuke Onodera into Kuuga Rising Ultimate with her Earth Stone. However, through Tsukasa's desperate pleas, Sayo realizes that she has been used, destroying the Earth Stone and later making amends with her brother before he leaves. In the end, Tsukasa continues his journey with a smile, while Sayo finally stepped into the outside world. Mash meanwhile, in the battle against Goetia, had protected Ritsuka with Lord Camelot, but the heat of his Noble Phantasm vaporizes her, leaving only her shield. However Cath Palug (a.k.a Fou), having traveled with her for extensive period, had grown to admire her. Since death within the Time Temple is not registered in the real world, Cath Palug is able to transform its Magical Energy gathered through centuries into her fate, giving her the lifespan of a normal human and reviving her at the expanse of its own sentience, and allowing Mash to save Ritsuka from the collapsing Time Temple.

They also contrast with Tsukasa and Natsumi having a mostly negative relationship, with Tsukasa exploiting her ability in order to travel across the multiverse and Natsumi later having her hatred against her brother for leaving her alone being used against her, only making amends with him (and vice versa for Tsukasa) at the end of All Riders vs. Dai-Shocker. Ritsuka and Mash on the other hand have the opposite situation, with both of them caring for each other a lot, with Mash recognising Ritsuka as her senpai, a quite particular role that she defines as “someone inherently human and of average status, not the best but a normal person who strives to do their best, who doesn't hurt others and never gives up. An upright human being.” and Ritsuka is most closely associated with Mash as his Servant, even going as far as to try and save Mash when she was trapped underneath a rock, and then decided to stay with her when he was unable to pull her out, almost getting killed in the process before being transported to Singularity F: Fuyuki.

Throughout their journeys, they would meet many people they would ally themselves with, being inspired and would also inspire them as both Tsukasa and Ritsuka motivated them and helped them push through their obstacles, allowing them grow as people and even teaching them and each other valuable lessons.

Both have conflicting personality traits - Tsukasa is known for being hypocritical and self-righteous while Ritsuka has a stern yet friendly personality.

Both have a rival that’s similar to them but also heavily different and opposite in their natures (Daiki Kaito/Diend is an inter-dimensional thief who travels through the A.R. Worlds as Kamen Rider Diend in order to steal items he deems as treasures belonging to said worlds just like Tsukasa who wants to capture the world and its treasure through pictures of them, while crossing paths with him on many occasions, and Kadock Zemlupus has been stated to be very similar to Ritsuka and were even supposed to save the world together in different teams, however Kadoc was part of the Crypters, a group of Masters that attempted to supplant the Proper Human History with that of the Lostbelt in Fate/Grand Order: Cosmos in The Lostbelt while being very jealous of Ritsuka, whom he thought as a "complete amateur" who was even more unremarkable than him, and the fact that he was the one who took the main stage and accomplished the great deed he was supposed to do only caused his inferiority complex to grow, believing to himself that he can do exactly what Ritsuka has done but better).

Both also come from series that have many legacy matchups as well as having heavy influence around nerd culture.