One of the main characters could be Shiro. Shiro has always been like a big brother to Keith. So when Keith turns into a girl, Shiro would be there to help him adjust. He'd make sure Keith is safe and comfortable in his new form. Also, Hunk could be a main character. Hunk would probably be the one to offer emotional support and cook some comfort food for Keith during this tough time of adjustment.
Well, in addition to Keith, there could be a new character introduced. Maybe a wise old woman who has knowledge about the kind of transformation Keith has undergone. She could guide Keith through the process of understanding and accepting his new form. And then there are Keith's enemies. They might not recognize Keith in his new form at first and this could lead to some interesting plot twists.
Obviously, Keith himself in his baby form would be a main character. His friends like Lance, Pidge, Hunk, and Shiro would also be main characters as they are the ones who would most likely be taking care of him or trying to find a cure. Maybe Allura and Coran could also be important characters, helping out with their knowledge of Altean technology or magic that might be related to Keith's transformation.
Well, perhaps the story could start with Keith being exposed to a strange energy. This energy alters his DNA, turning him into a girl. As a girl, he finds that he's now treated differently in society. He uses this experience to understand the struggles that girls face. He might also have to adapt to physical changes like different body strength and new ways of expressing emotions.
Well, the plot could involve his friends or comrades having to take care of him as a baby. Maybe they struggle to figure out how to feed him the right things or keep him entertained. For example, they might try different alien foods to see what baby Keith will eat.
I'm not sure of the exact title, but I once read a fanfic where Keith turns into a girl and it was really interesting. It had a lot of character development for Keith. The story started with the sudden transformation and then went on to show how he coped with things like having to wear different clothes in the Voltron base. His interactions with Lance also changed in a really unique way, as Lance was initially quite flustered by the change but then they developed a deeper bond based on mutual understanding.
Keith will likely be defiant. He has a strong sense of justice and will not be easily broken by Zarkon. He might try to fight back immediately if he has the chance.
Obviously, Keith and his mom are the main characters.
One common theme is identity. Keith has to figure out who he is now that he's in a different body. Another theme is acceptance, both self - acceptance and acceptance from the team. For example, in many of these fanfictions, Keith struggles with accepting his new form at first, but as the story progresses, he starts to embrace it. And the team also has to accept this new version of Keith.
A plotline could be that Keith is an undercover agent or spy, and being a girl would put her at risk in her mission. So she pretends to be a boy. As the fanfiction unfolds, she has to balance her real self and her false identity while also dealing with the challenges of her mission and the people she meets during it.
The team as a whole would become closer as they united to take care of Keith. His transformation into a baby would bring out different sides of each character. They'd have to rely on each other more, communicate better, and work together in new ways. For example, they'd need to share babysitting duties, which would lead to more interactions and a deeper understanding of each other.
I'm not sure specifically which 'keith is secretly a girl' fanfiction to recommend. You could try searching on popular fanfiction websites like Archive of Our Own. There might be some interesting stories there.