[Ashterot the Grimmsnarl] and [Nyrgal the Ditto] are both cut from the same bombaclaat.

Nyrgal's assholery stems from trust issues and a general disdain for humanity due to his experiences in the custody of Pokemon Hunters - who abused him in a multitude of ways, and later by mad scientists - who saw him as nothing but a test subject, which would later result in his then-unstable physiology and developing his Social Darwinian mindset - partly believing that his capture occurred because he was weak and strength was a necessity for survival. For any type of training to occur, Elio had to prove his own power and demeanor to Nyrgal on multiple occasions for the Pokemon to slowly warm up and willingly follow his commands.

Though, fortunately, Elio had a head start in gaining Nyrgal's trust after the latter witnessed Elio's one-man campaign against the same group of Hunters who once tormented him - even going as far as to offer the Ditto a chance at vengeance, which he gleefully took. In fact, their bond developed by leaps and bounds whenever the two eliminated Pokemon Hunters or humiliated arrogant individuals who abused others through their position - with Nyrgal gaining satisfaction out of destroying those in power rather than helping those weaker than him. Regardless, even when the two became close enough for Nyrgal to follow Elio's instruction, he was quite selective about what he learned and who his training partners were - preferring to follow his own path to indomitable strength and wanting to cause as much mischief as possible, staying true to his <Impish> nature.

On the other hand, Ashterot's assholery stems from his (well-founded) inflated ego - which frequently clashes with his oddly eloquent mannerism yet complimented his barbaric fighting style; he's self-sustaining and really didn't need a trainer to draw out his power. In fact, Elio considers himself to be Ashterot's trainer by technicality due to meeting the Nigredo standards, the latter finding him rather interesting via sensing Elio's cultivated power and becoming well-acquainted with the former's desire for freedom, and later being impressed with Elio's lavishness stemming from his self-cultivated financial status - drawing out his own desire to enjoy the finer things life has to offer.

While Ashterot's Nigredo strain is one of the few that's highly concentrated - thus explaining his robustness, imposing size, inverted color scheme, advanced intellect, other notable biological features (i.e. his hair follicles, fangs, muscle density, etc.), and extensively heightened development rate, after much observation and interaction - usually with the two being at odds with each other to certain degrees, Elio's decided that training him would be fruitless as the Ogre would produce astounding results if left to his own devices - something that's proven time and time again, with Ashterot frequently performing feats outclassing even the most well-trained veteran Pokemon after undergoing hours of self-discipline, replication, and extreme physical training. Such drastic leaps in power also stem from his humiliating experiences dealing with [Heim the Alolan-Ninetales], [Rowen the Metagross], [Lucifel the Wiseraph] and [Orochi the Serperior] - severely crushing pride while, in turn, spurring his drive to surpass and humiliate them to establish his "dominance".

J briefly had a Hydreigon in Unova - it was quite infamous in the area he found it in for its unruly and vicious temperament. Even when trying to appeal to it, the Hydreigon would lash out against J's other Pokémon and even try to attack him. It came to light very quickly that the Pokémon simply did not wish to live its life alongside a trainer, and was best suited to living in the wild. J was quick to accept this, and released it back where he caught it originally. In terms of Pokémon that he still has, his Froslass certainly took a lot of time to train. J was very patient with Naoko and made it clear to her that he was prepared to wait for as long as she needed to feel comfortable with proper training, and eventually she came around. She's now one of J's most formidable and reliable Pokémon partners :)

For Odd, definitely Mauve. Mauve's can be haughty/overly critical in the present, but it was even worse when she and Odd first met and their lives got tangled with each other. Though while she started off a little prickly and suspicious of him as a boy, she eventually softened on Odd after he kept trying to be hospitable through her grumpiness, and began to legitimately trust him after he helped her through some episodes of her getting blindsided by her dormant powers from her time as Seidallah, at a time when she didn't know about that aspect of herself.

(And I would've said Giratina, but neither Odd nor Gira themselves consider their relationship of the standard trainer/Pokemon dynamic. And even if they did, the process between them would be more akin the them training each other so they could both exist in relative harmony without hurting each other.)

That'll probably have to go to his Charizard.
Ever since he was a Charmander he was nothing but trouble, constantly picking fights, disobeying Trainers, and snapping at everything that annoyed him. Basically imagine Bakugo from MHA

Tyler saw potential in Charmander, and after rescuing him from Team Rocket, he gained enough trust for Charmander to be caught, but he was still as grumpy as ever

Charmander would listen to Tyler as it grew up into a Charizard, but it was still really rebellious, it still is honestly. But Tyler and him have really bonded over the years, even if he doesn't show it

The best example of this was when Tyler mastered Mega Evolution with Charizard