For Goku, you can focus on his growth as a fighter but also his naivete in relationships. Show how he reacts to the female Saiyan's presence, like his confusion about new feelings. For the female Saiyan, give her a distinct personality. Maybe she's more reserved than Goku in some ways but has a fiery side when it comes to fighting. Describe her backstory, like how she was trained on the Saiyan planet and what her goals are. You can also have them learn from each other. For example, Goku teaches her about the joy of fighting for the right reasons, and she teaches him about Saiyan traditions he may not be aware of.
In developing Goku, emphasize his unwavering dedication to becoming stronger. But also show how he starts to make room in his heart for the female Saiyan. He might start to think about her safety during battles more than he used to. For the female Saiyan, create a complex character. Let her be strong - willed and independent, yet also vulnerable when it comes to matters of the heart. You could have her struggle with adapting to Earth's culture while also trying to fit into Goku's life. They can grow together as they face challenges, like when a powerful villain attacks and they have to rely on each other's strengths and weaknesses to win.
To develop Goku, you can explore his simplicity. He loves fighting and food, but now there's this new element - the female Saiyan. Show how he starts to daydream about her between training sessions. For the female Saiyan, give her a backstory filled with challenges. Maybe she was an outcast on the Saiyan planet because she didn't conform to the traditional Saiyan ways. When she meets Goku, she sees a different way of being a Saiyan. They can bond over their shared Saiyan heritage but also their differences. As they go on adventures together, their characters can evolve. Goku can become more in tune with his emotions, and the female Saiyan can become more open - minded about different ways of life.