To develop her character, her flaws are important. Maybe she's over - confident at times which leads her into trouble. This can create interesting story arcs where she has to learn humility. Also, her cultural background within the Bionicle world can be explored. If she comes from a more peaceful tribe and now has to adapt to a more war - like environment, it can shape her values and actions.
Her interactions with other characters, especially non - Toa beings in the Bionicle universe, can develop her. For example, if she befriends a small, timid creature that others overlook. Through this friendship, she shows her compassion and also learns about the underbelly of the Bionicle world. Her physical appearance can also reflect her character, like if she has a unique mask or weapon that has a story behind it, which can be gradually revealed as the fanfiction progresses.
The new girl Toa can be developed by giving her unique powers. For instance, if she has the power to communicate with nature in a way other Toa can't. Her relationships also play a big role. If she has a mentor, like an older, experienced Toa, she can learn from them and grow. This relationship can show her naivete at first and then her increasing wisdom as she trains.