Base their actions on their motives. If Ichigo is fighting to protect his family and friends, show that in his every move. Fem Ichigo could have a similar motive but also something unique to her, like proving herself in a male - dominated world of fighters. By having clear motives for their actions, the characters will seem more real and engaging to the readers.
To create believable characters, study the original Ichigo's personality traits. He's brave, determined, and has a strong sense of justice. For fem Ichigo, you can keep some of those core traits but also add some feminine touches. For example, she might be more in touch with her emotions but still be as fierce in battle as Ichigo.