Sceptile Leaf Blade
Nighttime Guardian
Depends on how you define succeeding I guess. Most companion characters grow a lot and leave different from how they entered (not you Brock). Becoming the best of the world or region or whatever is to most people not really a realistic goal in the first place, and they generally do get reasonably close to that (not you Misty). There's something humbling and relatable about the message that even if you don't win literally everything it doesn't mean you're a failure (which some people tend to forget, especially when talking about Ash). Even making it to the Grand Festival, to the League, to the Master Class, or growing to learn how to use Z-Moves, these are all formidable achievements for characters that shouldn't be easily brushed aside, especially for a character like Lillie who started out not even being able to touch pokémon, and now she's capable of using a Z-Move with her own pokémon and entering a pokémon league.To be honest almost anybody could work since nobody succeeded at their goals.
I always wondered why the characters with clear goals aren't allowed to achieve their dreams? It's not like we are going to see them again with the exception of small cameos. Especially Serena and Iris who didn't even get cameos at all.
It kind of leaves you hanging...