In comics, Superman is usually stronger. His powers have been developed and refined over the years, and he often demonstrates greater physical strength and abilities compared to Supergirl. However, it can vary depending on the specific storylines and plot developments.
In some comic versions, Supergirl has married various characters, but it varies greatly depending on the creative direction of the writers. Sometimes it's a fellow superhero, other times it might be a civilian.
Well, Cisco Ramon from The Flash would be a great addition. He could use his tech skills to create some new gadgets or figure out ways to counter the villains' powers. Also, Martian Manhunter from Supergirl's side. His shape - shifting and telepathic abilities would add a whole new dimension to the story.
Well, obviously Kara herself is a common character. Then there's often her adoptive family, like the Danvers. Alex Danvers, her sister, is frequently in these fanfictions. They usually have a strong bond that's explored further. Also, Clark Kent might make an appearance. Since he's also from Krypton, he can act as a mentor or a source of information for Kara about their home planet.
Hinata could be one of the main characters. Since she has deep feelings for Naruto in the Naruto universe, she might follow him into Supergirl's world. Also, Kara's friend Lena Luthor could be involved. She is a complex character and her interaction with Naruto could add an interesting dynamic to the story.
Well, one common antagonist is Reactron. His radioactive powers can be a match for Supergirl, and in a dark supergirl fanfiction, he could be even more of a threat as she might be dealing with her own internal struggles. Another could be the Cyborg Superman. His combination of kryptonian and cybernetic powers makes him extremely powerful. He could be a great antagonist for a dark Supergirl as he might try to use her for his own purposes or simply see her as a rival. Also, the Phantom Zone criminals can be antagonists. They are escaped convicts from Krypton's most dangerous prison, and a dark Supergirl might have to deal with them while also trying to come to terms with her own dark side.
Supergirl herself is of course the most important character. Then there's often Superman, her cousin, who can play a significant role in her story, either as a mentor or as someone she compares herself to. Also, her friends on Earth, like those she meets at school or in her daily life, are important as they represent the normal part of her world that she tries to protect.
Supergirl could potentially win. She has the same powers as Superman under a yellow sun. Her heat vision can be extremely powerful. If she uses her speed and heat vision in a coordinated way, she could overpower Wonder Woman. Also, Supergirl might be more adaptable in different environments as she has had to face various threats on Earth and in space.