In some fanfictions, there could be original characters created just for the story. These characters might interact with Ben and his 'babies', either as friends, foes, or just bystanders. But overall, Ben and the 'babies' remain the key players in this type of fanfiction.
Obviously, Ben Tennyson is one of the main characters. He is the central figure around whom the story revolves. And the 'babies' are also main characters, though who they are exactly can vary. They could be new alien forms that Ben discovers and takes under his wing, or they could be some sort of younger versions of himself or other characters related to the Ben 10 universe.
Another plotline might involve Ben protecting his 'babies' from an enemy that specifically targets them because of their relation to Ben or their potential power. He'd have to use his experience and the various alien forms at his disposal to keep them safe.
In these fanfictions, both Ben and Juniper are central to the plot. Their different backgrounds, Ben from a more science - fiction based world and Juniper from a magical world, create a contrast. This contrast is often explored, whether it's in how they approach problems, how they communicate, or how they form a bond. They are the two main forces that drive the story forward.
Obviously, the Big Chill babies would be the main characters. They are likely to be the focus of the story.
The main characters are Rook and Ben's babies. Rook, who might be from a different background or with different abilities, is the one who is going to interact with the babies. The babies, on the other hand, could have inherited some of Ben's traits. Their interactions will form the core of the story. For instance, if Ben has superpowers and the babies show some signs of those powers, Rook will have to deal with that while getting to know them.
Another possible antagonist could be an alien scientist who was involved in the creation or discovery of the Necrofriggian babies. This scientist might have his own evil plans for the babies and sees Ben as an obstacle in his way. He could be using his advanced technology to try and capture the babies from Ben.
Well, 'Star - Crossed with Ben Tennyson' is a must - read. It has an interesting take on the relationship, with the reader character coming from a different world and how they and Ben deal with the cultural differences. And 'Love Among the Aliens' is really good too. It shows a more mature side of the relationship between Ben and the reader, dealing with issues like trust and loyalty in a very engaging way.
Obviously, the Big Chill babies would be the main characters. There might also be some other characters from the Ben 10 universe like Gwen or Grandpa Max who could play supporting roles.
In some fanfictions, baby Ben might be in a situation where he's being protected by his future self's allies. For example, Gwen and Kevin could be babysitting baby Ben, and they have to deal with all the chaos that his unpredictable Omnitrix activations bring. It's a fun concept as it shows a different dynamic between the characters.
To write a good 'Ben Tennyson x Reader' fanfiction, you need to focus on their relationship development. Start with how they meet and gradually build up the chemistry between them. Make use of the Omnitrix and Ben's alien forms in the story to add some action and excitement. For instance, the reader could be in danger and Ben has to transform into one of his aliens to save them. And don't forget to add some emotional depth to their interactions.
In this fanfiction, it could be that the Necrofriggian babies are the result of some sort of experiment gone wrong. Ben stumbles upon them and realizes that he is the only one who can help. He has to figure out how to reverse the effects of the experiment while also dealing with the fact that the babies are in a very fragile state. He might enlist the help of his friends, but also face challenges from those who don't want the experiment to be reversed.