Since we have a similar thread for Shooti, I thought it would be good to have a place to discuss what you like or dislike about Bel both as a rival and as a character. So, fire away!
For the most part, I feel that Bel brings a lot to the Best Wishes series. She's incredibly likeable and compelling, from the way she cares about her pokémon to the way she's always amazed by the things she sees and experiences in her travels. I also liked the way she seemed to want to get stronger after her last appearance and strived to become a better and more dedicated trainer. She's really someone that I always want to see more of and a character who's always easy to root for. It doesn't hurt that she's also incredibly entertaining and brings instant chemistry with Satoshi and Dento. The three of them really feel like a trio of really good friends and she just really works well with both guys.
I'd like to see her be more of a threat as a trainer, though after her last appearance, I don't doubt it will happen.
Chillarmy, but she's rash and sometimes a bit ditzy, but I fail to see how being a slave to your childish pokémon is a sign of belonging to Mensa. Satoshi's Pokabu not evolving has nothing to do with Bel's Chaobu since the writers have time and time again evolved pokémon that Satoshi shared with his rivals (Jun owning a Mukuhawk, Shinji's Dodaitose not hampering Naetle's evolution, etc.) How are Chaobu and Chillarmy any more or less believable than Gothimu? Besides the game character aspect, all the "flaws" you found in Bel (which some aren't factually backed by the animé) are also present in Koharu, except she displays not of Bel's good points.
I'm sorry but Bel has the storyline, she had to get over her father's judgement, and now she's really trying to show her worth as a trainer by actively training her pokémon. I fail to see how that makes her boring, especially when she's highly energetic and provides great comedy.