One idea could be that the Saiyan male OC is a warrior from a different part of the Saiyan planet. He meets Kale during a universal tournament. At first, Kale is shy and reserved around him, but as they train together for the next rounds of the tournament, they start to develop feelings. The OC could teach Kale some new Saiyan fighting techniques that she hasn't seen before, which helps her gain more confidence in herself not just as a fighter, but also in her relationship with the OC.
The Saiyan male OC can be developed by giving him a moral code. For instance, he might believe in always fighting fair and never using dirty tricks. This can contrast with Kale who, in her Berserker state, might not be as in control. As for Kale, you can develop her character by showing how she adapts to new situations. When the OC introduces her to new Saiyan traditions or places, how does she react? Does she embrace them or is she hesitant? Their characters can grow as they influence each other. The OC can inspire Kale to be more confident in her normal state, and Kale can teach the OC about the raw power that lies within a Saiyan when pushed to the limit.
To develop their relationship, start with small interactions. For example, they could exchange fighting techniques. This would allow them to see each other's strengths and respect each other. Then, put them in situations where they have to rely on one another, like being trapped in a dangerous place. As they work together to escape, their bond will grow. Also, have them share their pasts. The Saiyan can talk about his Saiyan heritage and Android 18 can share her experiences as an android. This will create a deeper understanding between them.
There are several common plots. For instance, the Saiyan OC male might be part of a team of warriors. They go on various missions together, and he has to learn to work with others, which is a challenge for the often - proud Saiyans. Another plot could be that he discovers a hidden Saiyan legacy. This could involve finding ancient Saiyan artifacts or learning about a long - lost Saiyan technique that changes his life and the course of the story.
A common theme is the power - up journey. With Caulifla and Kale being Saiyans, many fanfictions focus on how an OC can either help them in their power - up quests or be inspired by them. Another theme is friendship. The OC might form deep bonds with Caulifla and Kale, going through various adventures together and relying on each other.
First, focus on his physical traits. A Saiyan male is often strong and muscular. Give him a unique hair style, like the spiky Saiyan hair but with a different color perhaps. Then, think about his personality. He could be proud like most Saiyans, but also have a soft side. For example, he might be kind to his friends but still eager to prove his strength in battles.
One plot idea could be that the OC male Saiyan is initially at odds with Android 18. Maybe he's on a mission to stop her from causing chaos, but as they interact more, he realizes she's not as bad as he thought. Another could be that they team up to face a common enemy, like a powerful new villain threatening the Earth. And they gradually develop feelings for each other during the battles and shared experiences.
You can try looking on fanfiction websites like Archive of Our Own (AO3). There are a large number of writers who create stories in this category. Just search for 'oc x caulifla x kale' in the search bar, and you'll be presented with a variety of fanfictions to choose from.
Power - up is a common theme. Male saiyan OCs often strive to increase their power levels, like through intense training or unlocking new forms. Another theme is honor. They usually uphold the Saiyan's sense of honor in battles and interactions.