Cynthia is definitely an interesting character to write about. She's the champion in Sinnoh and has a lot of mystery surrounding her. You can explore her past, her training methods, and what makes her such a powerful trainer. Another character is Barry. His over - eager and sometimes clumsy nature can lead to some really fun and comical storylines in fanfiction.
One interesting character is Looker. He has this detective - like vibe in the games, and in fanfiction, you can expand on his investigations in the Sinnoh region. Maybe he's on the hunt for a group of poachers or trying to solve a mystery related to a rare Pokemon. Professor Rowan is also a good character to write about. You can explore his research more deeply, and how it impacts the trainers and Pokemon in Sinnoh.
Cynthia is also very popular. As the Sinnoh region's champion, she is a powerful and mysterious figure. In fanfics, she can be portrayed as a mentor, a rival, or even an antagonist in some unique storylines. Her Gardevoir and Garchomp are also often featured in these stories.
One popular character to include is Giratina. Even though it's a Legendary Pokemon, it can be given a more complex personality in fanfiction. Maybe it has a reason for being so feared and banished to the Distortion World that goes beyond what is shown in the games. Also, Fantina, the Gym Leader of Hearthome City, with her unique style and love for Ghost - type Pokemon, can be a great addition to a fanfiction. And don't forget about Maylene, the Fighting - type Gym Leader. Her journey to become stronger and her relationship with her Pokemon can be explored in depth.
One idea could be exploring an alternate universe where the main characters have different Pokemon partners. For example, instead of Dawn having Piplup as her starter, she could start with a Chimchar. Another plot could revolve around a mysterious new Pokemon that only appears in a long - forgotten part of the Sinnoh region. It could be a Pokemon with unique powers that both the good and bad guys are after. Also, a story about a trainer who travels through Sinnoh backwards, starting from the Pokemon League and working their way to the beginning of their journey, facing challenges in reverse order.
One of the most popular characters is often the main trainer. They are usually the ones going on the adventure, facing challenges, and growing throughout the story. Another popular character can be the rival. Their competitive nature and growth parallel to the main trainer make them interesting. Legendary Pokémon also tend to be popular characters in these fanfictions as they bring an air of mystery and power.
A great storyline might involve a group of friends who each have different goals in the Pokemon Diamond and Platinum world. For example, one might be focused on winning all the gym battles, while another is more interested in finding and documenting rare Pokemon species. Their different paths cross and they end up helping each other in unexpected ways.
One popular theme is the journey of a new trainer in the Sinnoh region, which is the setting for both games. They might face unique challenges and form special bonds with their Pokemon. Another theme could be about legendary Pokemon, like exploring the backstories of Dialga and Palkia in fanfiction. Also, fanfiction often focuses on the rivalries between trainers, adding new twists to the in - game rivalries we see in Platinum and Diamond.
The main character from the game, of course, is a popular choice. Also, Cyrus from Team Galactic is often included as he is such a complex and interesting villain. And Dawn or Lucas, depending on the version, are great to have as they are the typical Pokemon trainers that players can relate to.
I think Oleana could be an interesting character for a fanfic. She's so loyal to Rose, and there's a lot of mystery surrounding her. A story could be about her past and what led her to be so dedicated to Rose. Also, Sonia is interesting. As a Pokemon researcher, there are many things to explore about her, like her research on the Galar region's Pokemon and her connection to the professor.
For me, the Team Star bosses are quite interesting characters for Pokemon Scarlet and Violet fanfic. They have their own reasons for forming the group and rebelling against the school system. You can write about their past, what led them to this point, and how they interact with the main character. Penny is also an interesting character. She is a bit of an enigma at first, and writing about her true identity and her role in the events of the game can make for an engaging fanfic. Also, Professor Turo or Sada (depending on the version) can be fascinating to write about. Their research, their relationship with the player character, and the secrets they hold about the Scarlet and Violet region are all great elements to explore.
Another character that might show up often is Professor Rowan. He could be involved in Ash's journey in various ways, such as providing information about new Pokemon or helping Ash with research. And of course, the Pokemon themselves are like characters. The Sinnoh Pokemon like Piplup, Chimchar, and Turtwig are common in these fanfictions as they are associated with Ash's journey in the 'pokemon platinum' story.