Fans of the Highschool DxD often ponder about the romantic development between characters Issei and Rias. The growth of their relationship can be tracked through various volumes of the Light Novel series, a popular form of narrative in Japan.
Initial Relationship Development
Issei and Rias‘ bond begins to deepen romantically around Volume 22 of the series. On a rather notable occasion, despite being on a date with someone else, Rias shows her trust in Issei, convinced of his love for her. Afterward, Issei proposes they talk casually during their private moments, which Rias happily agrees to.
This marks a significant milestone in their relationship.

Issei’s Promotion to High-Class devil
Still in Volume 22, Rias plays a vital role during Issei’s promotion to being a High-Class devil. A significant event during the ceremony was Rias placing the crown onto Issei’s head, which marked him as a King.
The Proposal
The development takes a leap when Issei, after Rias’ graduation ceremony, proposes to her; thereby marking her as his first bride-to-be. This important event takes place in the later part of Volume 22.

Multi-Marriage Plans
In an unexpected twist, Volume 25 reveals Issei’s plan to marry all eight girls in his life, implying a multi-marriage. Rias, along with Asia, Xenovia, Irina, Akeno, Kuroka, Koneko, and Rossweisse each are shown to be part of his plan, including his intention for intimacy with all of them.
Fierce Rivalry
Even with their engagement, they both still are competitive. In True Volume 1, while facing each other in a match, they promise to give their best, with Rias even joking about calling off their engagement if Issei doesn’t maintain intensity during the competition.

Conclusions
In the whirlwind of a fantasy series, High School DxD, the character development and the multi-faceted plot create an engrossing story. One of the most captivating subplots is the romance between Issei and Rias, which moves from casual friendship to a complex, multi-marriage plan. Now that the question “What volume does Issei marry Rias?” is finally answered, fans can further appreciate the depths and nuances of their relationship. The journey from Volume 22 onwards provides an emotional roller-coaster for readers and followers of the series.