Count Dooku could be a prominent character. Given his own fall to the dark side, he might be a mentor or a rival to Obi - Wan in the fanfiction. Mace Windu is another one. His strict view of the Jedi Code and his relationship with Obi - Wan could be explored in the context of Obi - Wan's turn. And of course, there are the various Sith Lords and dark side adepts that might surround Obi - Wan, either as allies or enemies in his new dark side life.
The author of the 'obi wan kenobi novel' is not something I can recall immediately. It would be a good idea to search for it in a library database or on a dedicated book - related website.
Jon Snow could be one. He has a sense of honor and leadership, much like Obi - Wan. They could bond over their shared values and Jon could learn from Obi - Wan's wisdom. Another could be Daenerys Targaryen. She is on a quest for power and Obi - Wan might see her as someone who needs guidance to use her power responsibly.
Ezra Bridger would be a great character to pair with Obi - Wan. Ezra is still learning about the Force, and Obi - Wan could be his wise mentor, teaching him more advanced techniques and the true nature of the Force.
Well, Obi - Wan Kenobi would obviously be one of the main characters. Depending on the nature of the love story, if it's about his relationship with Anakin, then Anakin Skywalker would be another key character. If it's a romantic love story, there could be a female or male character that is the object of Obi - Wan's affection, perhaps a fellow Jedi, a senator, or someone from a planet he visits.
The twins could be new characters created specifically for this fanfiction. They might be force - sensitive individuals with a great destiny ahead of them.
There may not be mainstream, widely - publicized fanfictions with Obi - Wan gagged. But some independent writers might have created stories around this concept. It could be that in a story, Obi - Wan is gagged during a diplomatic mission turned hostile. His captors want to silence him to gain an advantage in negotiations. This could lead to a complex plot where his friends have to use their wits and skills to free him and resolve the diplomatic crisis.
Maybe the Force itself could be an antagonist in a sense. The light side of the Force would be trying to pull Obi - Wan back from the dark side. And the natural balance of the Force that doesn't want too much dark side influence could be working against Anakin's plan to turn Obi - Wan. So the Force, in its own way, could be the main antagonist.
Obviously, Obi - Wan Kenobi is one of the main characters. There might also be characters like Anakin Skywalker, as their fates are intertwined. And perhaps some Jedi Masters or other Jedis from the Order who interact with Obi - Wan.