And then there's just Trip from BW who was unpopular and completely forgettable.
Well he was not only forgettable, he was flat out one of the most unpleasant characters in the anime. Dude was an arrogant and xenophobic/racist prick who had an unbelievably massive ego despite being a new Trainer and was condescending towards everyone for no reason, even though he had nothing to back himself up. He was so full of himself at some point that he arrogantly thought he could beat Cynthia and Alder and basically made it clear he thought he was above them (even having a smug grin on his face when approaching them). Boy, that is delusion at its finest. Good thing Alder took him down a couple of notches afterwards. Dude barely had any genuinely compelling battles with Ash or a real dynamic with him other than Trip calling Ash a hillbilly and calling Kanto the boonies, which does nothing but make me despise Trip overall.
Dude was an unpleasant and downright contemptible sight of a character and he was the one person in the anime that I genuinely dislike. Like sure, Paul may not have a likable personality and was an asshole, but he was a genuinely strong and experienced Trainer who had very different views from Ash in how to best train Pokémon to become strong, but a similar level of experience, and the constant differences between them created a genuinely compelling rivalry. Plus Paul's obsession with strength is revealed to be driven by his disappointment in his brother and determination not to fail like he did. Not to mention, the dude may have been harsh and stern with his mons, but he had the bite to back up his bark and unlike Trip, he at least knew his limits and respects strong Trainers like Cynthia and acknowledges other's strength when they prove themselves worthy. Gary Oak on the other hand may have also been full of himself but he at least had popularity to back himself up: like the guy
had a car and had an entire posse of fangirls at his behest, and was "cooler" than Ash and was always a step ahead of him, so again, he had something to back his boastfulness up, and combined with the fact that the two had a contentious relationship to begin with that also created for a genuine and contentious rivalry, especially when it ended off at the Johto League and their relationship changed.
Trip genuinely had nothing to make himself a really compelling rival to Ash in any way, and unlike Gary and Paul, he certainly had nothing to back up his assholery or his massive ego. It's like they
tried to hit similar notes as both Gary and Paul with Trip but failed miserably at replicating what made either of them good rivals to Ash.
Trip is genuinely the one character where I'd be happy if he never shows up again ever in the anime. Unpleasant personality that made him a pain to watch, dude certainly had no bark to back up his bite as anyone who wasn't Ash crushed his ass most of the time (and this only served to make BW Ash look even worse than he already did), and the dude never had any truly compelling dynamic with Ash as a rival. He's the character I remember for the fact that I dislike him quite a bit honestly as both a person and a character.