Krillin’s relationship with Android 18 in the Dragon Ball universe is certainly less than conventional but isn’t as surprising as some fans may think as their marriage was seemingly foreshadowed upon Krillin’s first appearance.
Krillin is one of the most powerful martial artists on Earth as he studied alongside Goku under Master Roshi in the early days of the Dragon Ball manga. During their time training together, Krillin and Goku learned many of the same techniques, including the incredibly powerful ki blast, Kamehameha. While they helped each other grow in power, Krillin and Goku also grew their friendship and, as they reached adulthood, they became the unofficial protectors of the planet known as Z Fighters. Years later, Krillin and Goku immediately answered the call to action by Trunks, the time traveling son of Vegeta, as he warned all of the Z Fighters of the impending threat of the Androids. In Trunks’ future, Android 17 and his sister, Android 18, decimated the Earth, so Trunks went back to ensure that wouldn’t happen. While battling the Androids after they finally made their presence known, Krillin fell head-over-heels for one of the villains, Android 18. In fact, Krillin felt so strongly for the Android that after she was absorbed by Cell and then regurgitated by the villain upon his defeat, he ran to her aid despite the fact that she had very recently tried to kill all of them–a moment that even Vegeta thought was incredibly foolish.
In Dragon Ball Chapter 25 by Akira Toriyama, readers are introduced to Krillin after the small warrior paddles his way to Master Roshi’s island and begs him to take him on as his student. Roshi agrees but only if he and Goku complete one task for him, and Krillin doesn’t hesitate to accept the offer. When it came time to leave the island and accomplish what Roshi had asked of them, Goku jumps on his powerful artifact, Flying Nimbus, so they can traverse the globe faster than they ever could without. However, Nimbus only allows people who are pure of heart to ride it, so when Krillin tries to jump on the cloud alongside Goku, he just falls right through. Given that his lack of a pure heart draws suspicion, Roshi asks Krillin if he had ulterior motives to learning advanced martial arts, and Krillin reluctantly admits that he really only wanted to learn how to fight so that he could impress girls.
The fact that Krillin only became the Z Fighter fans know him as today so that he could impress potential love interests completely explains his relationship with Android 18. While the other warriors of Earth saw the Androids as a world-ending threat that had to be eliminated at all costs, Krillin became lost in his attraction to the villain and almost helped destroy the world in the process. During the Cell Saga, Krillin had the chance to ‘kill’ her before 18 was absorbed by Cell which would have kept him from reaching his perfect form. However, Krillin just couldn’t do it which allowed Cell to grow in power exponentially. Then, when all the fighting was over, Krillin helped 18 to safety just so he could have a shot at dating her, which is a risky move given how dangerous she proved to be.
While Krillin is a committed fighter and one of the greatest protectors of Earth, his first appearance along with his relationship with Android 18 proves that his warrior spirit is not the driving force behind his battles like it is for Dragon Ball warriors like Goku or Vegeta. Krillin is a lover, and he only fights to either win love or strengthen the love he’s already won–something that was made clear upon his debut which perfectly explains his marriage to Android 18.