The girl could be a curious scientist - young and brilliant, always eager to explore new things. The man might be her colleague, who is a bit more cautious. Their different personalities would add to the story when the shrinking incident occurs.
The girl could be an ordinary schoolgirl with a strange family heirloom. One day, the man, who is her neighbor, comes to visit. She shows him the heirloom, and somehow it causes him to shrink. In this case, the main characters are the normal girl and the neighbor who suddenly finds himself in a very strange situation.
It could be an ordinary girl named Lily. She is just a normal school - going girl until the shrinking incident happens to her.
Perhaps in a fantasy world, the girl has a special power that she can't fully control yet. When she gets angry or excited, this power is unleashed and shrinks the man. The story could be about her journey to learn to control her power and also to help the shrunken man. She may have to take him on adventures in the miniature world he now inhabits, facing tiny monsters and other obstacles while trying to find a way to make him big again.
The main character is, of course, the girl who shrinks. There could also be her family members who are often clueless about her situation at first. Sometimes, there might be a friendly animal or a magical creature that becomes her guide or helper during her shrunken state.
The family pet could also be an important character. If the husband is shrunk, the family dog or cat might see him as a new toy or a strange little creature. This could create a lot of humorous and dangerous situations for the shrunken husband. And the wife has to constantly protect him from the over - friendly or curious pet.
The 'girl shrinks story' might be about a scientific experiment gone wrong. The girl is in a laboratory, and an experiment causes her to shrink. She then has to find a way to reverse the process while navigating the dangerous world of the lab at a minuscule size. There could be all sorts of obstacles like spilled chemicals that are now like giant puddles for her, and microscopic organisms that pose a threat.
The elf is surely one of the main characters as the story is centered around the elf shrinking. Santa Claus could also be a main character. He might be the one who tries to help the elf get back to normal size, or he could be the one who unknowingly caused the shrinking. Another possible main character could be a reindeer. The reindeer could be the elf's friend who helps him get around in the now - huge world since the elf is so small.
Maybe the shrinking girl is a new - comer to the city, and she was on the bus exploring the area when this strange thing happened. The girl who keeps her could be a local girl with a big heart, who often helps others and saw this as an opportunity to do a good deed.
The main character is obviously the girl who shrinks. She could be a young and adventurous girl, always eager to explore new things which is why she got involved with the potion in the first place.
Maybe the girl accidentally drinks a magic potion that has the effect of shrinking. She then has to face a whole new world full of challenges, like trying to get back to her normal size while avoiding being stepped on by others.
It could be a story where a girl has some kind of strange experience related to her feet while on a bus, like she feels her feet shrinking perhaps due to some physical discomfort or a very strange psychological perception.