Fiona Lemon is another main character. She might have her own unique set of skills or traits that complement Sonic's. She could be intelligent, brave, or have some special power that helps in their adventures together.
A third type of plot could be Sonic and Fiona Lemon getting lost in a strange and magical realm. They then have to find their way back home while also helping the inhabitants of that realm. In this journey, they learn more about themselves and their relationship. They might discover new powers or abilities that they use to overcome the obstacles in this unfamiliar place.
Obviously, Sonic and Amy are the main characters. Sonic, the speedy blue hedgehog, and Amy, the determined pink hedgehog. There might be other characters as well, but they are likely to be in supporting roles compared to Sonic and Amy.
Knuckles could also make an appearance in Sonic the Merhog fanfics. He might be guarding some underwater treasure or ancient relic. Sonic has to convince Knuckles that they need to work together to protect it from a common enemy, like a group of pirates who are trying to steal the underwater treasure.
Well, obviously the main characters would be the Powerpuff Girls - Blossom, Bubbles, and Buttercup. And Sonic the Hedgehog would also be a central character. There could also be other characters from both universes like Professor Utonium from the Powerpuff Girls side and Tails or Knuckles from Sonic's side depending on the direction of the fanfic.
There might also be new characters created specifically for the fanfic. These could be hybrids of the two universes or characters that are designed to bridge the gap between Shrek's world and Sonic's world. They could play important roles in the plot, like being the ones who bring Shrek and Sonic together in the first place or being the key to solving the problems that arise during their adventure.
Friendship is likely a common theme. The fanfics might show how Fiona and Emma support each other through various situations.
Knuckles might also be part of the story. As the guardian of the Master Emerald, he has a sense of responsibility. He could either be involved in the events that led to Sonic's fall or be part of the rescue mission. His strength and knowledge of the area (if Sonic falls in a location related to the Master Emerald) could play an important role.
Monika is often depicted as intelligent and manipulative in her original game context. In the fanfic, she might use her intelligence to analyze Sonic's world and help him solve problems. Sonic, on the other hand, is known for his speed, courage and his sense of justice. He would bring his adventurous spirit to their interactions.
The main characters in the original story of Princess Fiona are Fiona herself, of course. And then there's Shrek, the ogre who becomes an important part of her story. Also, there are various fairy - tale creatures like Donkey who play significant roles in the plot.
The main characters are clearly Jonas and Fiona. They are likely the focus of the story as the title implies that they are the ones who are pregnant (in the context of the fanfiction).