Gabriel's twin sister in the fanfiction could be a powerful force for good or evil. If she's good, she could be a key player in the fight against the demons and other dark forces. She might use her powers, which could include things like enhanced telepathy or the ability to teleport large groups of people, to help the hunters. On the other hand, if she turns out to be evil, she could be a new and formidable adversary. Maybe she was corrupted by the same forces that tried to use Gabriel, or she has her own reasons for going down a dark path. This would create a whole new dynamic in the Supernatural story.
Perhaps Kayla is the key to solving a major mystery in the Supernatural world. She might hold the knowledge or ability to defeat a powerful new villain that has emerged. This could lead to a series of adventures where she teams up with different characters from the show to uncover her true potential and save the day.
One possible plot could be that Gabriel's twin sister has been hidden away by their family for some reason. When she finally emerges, she has powers that are different from Gabriel's but equally as strong. She teams up with the Winchesters to fight against a new, powerful enemy that is threatening the world.
Kayla could be a fierce and independent character. Since she's related to Gabriel, she might have a similar sense of humor and a love for mischief. However, she could also be more empathetic and kind - hearted, using her powers to help those in need rather than just for pranks.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of Supernatural fanfics, and you can use the search function to specifically look for stories about Gabriel's twin sister. Another option is FanFiction.net, which also has a wide variety of Supernatural - related fanfictions.
A great plot could involve her having a prophecy associated with her. The prophecy could state that she will either bring about the end of all supernatural chaos or be the cause of its ultimate destruction. This would put her in a very difficult position. Everyone in the Supernatural world, from the angels to the demons to the hunters, would be interested in her. Some would want to protect her to ensure the positive outcome of the prophecy, while others would want to kill her to prevent the negative one. Along the way, she would have to learn about her true nature and powers, and make choices that would determine the fate of the entire Supernatural realm.
Some fanfiction authors develop her by giving her a backstory that involves her being an outcast among the supernatural beings. Maybe she was shunned because of her relation to Lucifer. This then shapes her into a more empathetic character who wants to prove herself different from her twin. She might learn about human emotions and values, which further differentiates her from Lucifer, who has little regard for such things.
Often, she's portrayed as a sort of mediator. In many fanfictions, she tries to reason with Lucifer, to bring him back to the side of good or at least stop his rampage. This can lead to some really emotional scenes between the two siblings. Also, there are stories where she falls in love with one of the regular characters in the Supernatural universe, like Sam or Dean. This love story then becomes intertwined with the overarching plot of good vs. evil.
Well, a popular Gabriel Supernatural fanfiction is 'Gabriel's Hidden Powers'. It's exciting as it explores powers of Gabriel that were not fully shown in the show. Then there's 'Gabriel and the Winchesters', which is all about his interactions with Sam and Dean. It has some really great dialogue that feels true to the characters. Also, 'The Real Gabriel' is a fanfic that tries to uncover the true nature of Gabriel, peeling back the layers of his character in an interesting way.
One great Gabriel fanfiction is 'The Trickster's Gambit'. It delves deep into Gabriel's character, showing his complex nature between his trickster side and his more serious, protective side. Another one is 'Gabriel's Redemption Arc' which focuses on his journey to make up for his past mistakes in the Supernatural universe. And 'A Day in the Life of Gabriel' gives a really fun and light - hearted look at what his daily existence might be like among the other characters.
One common theme is family. Since Sam has a twin sister in these fanfictions, it often explores their family bond, how they support or sometimes conflict with each other. Another theme is the supernatural world. The sister usually has to navigate the same world of demons, angels, and monsters as Sam.