Princess Peach could play a role. She could befriend the shy girl and introduce her to Mario. Peach could also act as a mentor to the shy girl, teaching her how to be more confident and ladylike in the Mushroom Kingdom. Another character could be Bowser. He could be the antagonist who tries to take advantage of the shy girl's vulnerability, which then forces Mario to step up and protect her.
Luigi could be an important character. He might notice the shy girl before Mario does and tries to encourage her to talk to Mario. Or, Toad could be there as a friendly sidekick, helping the shy girl get used to the new environment in the Mushroom Kingdom. He could also give her tips on how to overcome her shyness based on his own experiences.
Yoshi could be a main character. He could be the one that the shy girl first feels comfortable with because of his gentle nature. Yoshi could then help the shy girl communicate with Mario and the others. Additionally, Donkey Kong could be involved. He might initially seem intimidating to the shy girl but later becomes an ally, showing her that not everything that looks tough is actually mean.
A plot could involve the shy girl being a huge admirer of Mario from afar. She is too shy to talk to him directly but leaves little anonymous gifts for him. Mario eventually catches her in the act. At first, she's embarrassed but Mario is kind and encourages her to be more open. They end up going on an adventure together where the shy girl discovers her own hidden strengths.
Since I don't know the exact'shy girl expansion story', it's hard to say who the main characters are. But surely the shy girl herself is one of the main characters. There might also be her family members, friends, or a mentor - like figure who plays an important role in her growth.
The main character would obviously be the shy girl herself. Maybe there could also be her best friend who is the first to notice the strange butt expansion and tries to support her through it.
Suga could also be the main character as the doctor. Given his cool and sometimes aloof exterior, it would be interesting to see him interact with a shy girl. The girl could be a volunteer who is afraid to speak up. Suga's blunt but kind nature would eventually help the girl gain the courage to express herself more freely as he gets to know her better.
Another possibility is that a group of Koopa minions could be the main antagonists. They might be under the orders of Bowser, but they have their own little schemes going on. They could target Mario - turned - girl because they think she'll be an easier target in her new form, but they are in for a big surprise when she shows her strength.
Luigi would probably be very surprised at first. He might be a bit confused, but then he would support his 'sister' Mario. Princess Peach would be delighted. She could finally have a female friend who has all of Mario's skills. Bowser might not even notice at first, thinking it's just another ploy by Mario. But when he realizes it's a real transformation, he might be a bit more hesitant in his attacks, not sure how to deal with this new female Mario.
Princess Peach would be a great addition. She could be in trouble as usual, and Mario and Luigi have to rescue her. But this time, she could also play an important role in helping them solve a mystery or defeat a powerful enemy. For example, she might have some knowledge about an ancient artifact that the brothers need.
Well, one possible plot could be that Mario touches a magical item in the Mushroom Kingdom. This item has the power to transform him. As he turns into a girl, he discovers new abilities. Maybe he can now access areas only females can in the kingdom. He also has to deal with how his friends like Luigi react to his new form.
The main character is the camera - shy worm, of course. Another important character could be a friendly sparrow who befriends the worm. The sparrow doesn't understand why the worm is so afraid of cameras at first, but as they become closer, the sparrow tries to help the worm face its fear. There might also be a grumpy mole who accidentally digs up the worm's hiding places from the cameras.
A love interest who breaks the girl's heart can also be a main character. His actions, like cheating on her or suddenly leaving her without an explanation, can be the catalyst for her transformation into an evil character. Then the story could revolve around her trying to get back at him in devious ways.