His friends would probably take care of him. Midoriya would be very eager to help, using his kind-hearted nature. He would make sure Todoroki is fed and comfortable.
Well, in a Todoroki turns into a girl fanfiction, perhaps there's a villain who has a transformation quirk and uses it on Todoroki as a form of psychological warfare. Todoroki, now a girl, has to find a way to reverse the effect. In the process, he/she realizes that society treats female heroes differently and has to fight against those prejudices too. Maybe he/she even uses this new form to her advantage in some battles, by using the element of surprise.
Well, in this kind of fanfiction, probably his friends would be super shocked at first. They might struggle to take care of him as he's now a baby. His powers could also be affected in some strange ways, like they might be uncontrollable or show up in unexpected forms since he's in a baby's body.
One idea could be that Batman is turned into a cat by a villain's new experimental device. As a cat, he has to rely on his natural feline abilities like stealth and agility to still protect Gotham. For example, he could use his cat form to get into small spaces in buildings to gather evidence or spy on criminals.
His relationship with Midoriya might change. Midoriya could become more protective of Todoroki as a girl, seeing her as more vulnerable. With Bakugo, there could be a different dynamic. Bakugo might initially tease Todoroki more, but as he sees Todoroki still being strong in her new form, he might develop a new respect for her.
Well, it could be about a woman who, perhaps due to a magic spell or some kind of supernatural occurrence, transforms into a cat. Maybe she's a witch who accidentally casts a wrong spell on herself, or it could be part of a curse. There are many possible interpretations depending on the context of the story.
Well, he might find it really hard to operate his Bat - gadgets as a cat. His paws are not made for pushing buttons or typing on keyboards. So he has to figure out a way to adapt. Maybe he uses his cat claws to scratch out messages for Alfred to understand what he wants to do.
Well, often he has to figure out how to communicate. He might try to use his body language in creative ways to get his friends' attention. For instance, he could lead them to where danger is by pawing at their feet and then running in the direction they need to go.
In a 'ciel turns into a cat' fanfic, the story could start with Ciel waking up one day as a cat. He's confused and frustrated at first. But then he realizes that he can now explore places he couldn't before in his human form. He might end up befriending a group of street cats who teach him how to survive as a cat. Meanwhile, his servants are desperately searching for him, not realizing that he's the little cat they keep seeing around the manor.
Well, obviously Izuku is one of the main characters, even when he's turned into a cat. His friends like Bakugo and Ochaco are also likely to be main characters. They would be the ones trying to figure out what happened to Izuku and how to turn him back. And maybe All Might could also be a main character, if he gets involved in the whole situation to help his young protege.