What does your character[s] believe their best and worst traits are, vs what would someone close to them say their best/worst trait is?
Wes would consider his best trait to be his penchant for survival. Even besides the obvious blessings he's gotten; he can find or carve a notch for shelter in a cliff, he can eat pretty much anything, he's built up his own immunity to poisons, his pain tolerance is absurdly high. He'll include the skills he's learned to survive; fleecing people of everything they own, pickpocketing, fighting alongside his pokemon, being able to create any manner of explosives.
His worst trait is a lack of a moral compass. He'll do whatever the people he loves asks of him. For most of his life, that was Gonzap. When Team Snagem tried to make Espeon and Umbreon shadow pokemon, he defected and the person he followed became Rui. When Rui left, he was left drifting until an intervention happened and Wes was forced out of Orre and into Galar. For the first time, Wes had to try and guide his life himself, until Leon crashed into his life.
If the person he loves needed him, Wes will throw every bit of morality out the window.
And at night, under the cover of darkness, he will admit to himself that there's something within his core. It hungers for blood and violence and death, and Wes has to do his best to supress it or else risk becoming a monster.
Leon sees past all of Wes's angry feral layers, and sees a man who loves pokemon. Obviously, he betrayed the only family he knew because they threatened to hurt his closest family, Espeon and Umbreon, but he also cares so much about the pokemon he's purified. He lets the purified mons wander, and is immensely patient with all of them, even if he has scars from their attacks when they were still shadowed. Wes even cares for wild pokemon, protecting them from poachers, practically able to talk to them. His heart and his relationship with pokemon was one of the first things Leon saw, and what caused him to fall in love with Wes.
But to Leon, Wes's worst trait is his perception of himself. Of a monster who's done unspeakable things to protect those closest to him. He sees himself as a tool for someone else, and puts no value in himself as a person. He doesn't see himself as one.
So Wes doesn't take care of himself. He doesn't have self preservation, he'd willingly sacrifice his life for the people he love.
All Leon wants is for Wes to be able to see himself as Leon sees him.
If Leon was asked in an interview, he'd say that his best trait is his work ethos and perseverance. He'd never back down from any challenge, overcoming it with a single minded focus. As a child, Leon's focus was becoming Champion. For a long time, it was making the Galar League the best it can be. Now it was being the best boyfriend he can be.
He'd only admit it in the Champion group chat, but his worst trait is his workaholism. He can't take a break. He can't pick up hobbies. He struggled to do anything other than work or training with his pokemon. Of course, he'd never hold his pokemon to the same standards that he sets for himself, but if it's not about pokemon battling, he will struggle to focus on it.
Once he met Wes, the worst trait wasn't workaholism, but the intensity of his focus. Instead of his thoughts centering on pokemon battling, now it's on Wes. He now struggles to do his job as Champion because he'd rather be out in the Wild Area with Wes.
Wes would describe Leon's best trait as "being sunny." There was something in the man's character that made him glow. He showed such enthusiasm for life, even the most mundane things like rescuing a Meowth from a tree or for walks in the Wild Area. Just being around Leon made Wes's day brighter, and he could tell that it wasn't just Wes. Leon makes his own and Wes's pokemon happier, he makes Hop and his friends happier, he makes everyone around him happier. There was just something about Leon that made the world brighter, even in somber moments.
Unfortunately, his boyfriend's worst trait was being a rampant people pleaser. For the longest time, Leon overworked himself by taking on all sorts of jobs for Rose and Macro Cosmos. Wes can see how Leon's kindness was being taken advantage of by Rose, and he hates it.
If Leon can't tell people no, Wes will just have to do it for him.