This is based on the Pokemon Diamond and Pearl Adventure!! manga, to be specific. I don't own Pokemon.
If you say that "love at first sight" isn't real, Jun will simply say that he must be blind with love, thanks to Mitsumi.
Now, Jun is and always will be a ladies man. There have been plenty of girls he watched go by that he would allow his eyes to wander over, appraising their looks. He's no expert in the field of love, but he's not a beginner either. Passing flings come and go, and most were not even that, merely just watching a girl walk by and think to oneself, "Wow, that's one girl I'd like to take to dinner sometime."
But that day, from where he was sitting high above on steel girders, tossing a pokeball in the air with boredom, Jun spotted them, the other assistants for Professor Rowan. He hadn't met them before, but he had heard of them. He passed over with vague indifference over the energetic Pachirisu-like boy and found himself focusing on the girl next to him. Lean figure, zigzag-like green hair, short pink skirt, those eyes...
It took all of five seconds for him to realize that he was in love with her. It had only taken one moment, when her eyes briefly flickered in his direction without actually seeing him, for him to think, This is the girl I want to spend the rest of my life with.
So Jun introduced himself to the two of them. Albeit a little directly, having tried to hug her and all and declare that he would like nothing more than to be her boyfriend, which earned him getting slammed into the dirt. Even as painful as that was, it showed that she was strong, a quality he liked, so he didn't mind too much. As for the boy, Hareta, he honestly didn't have one opinion over another, other than having respect for him as a trainer. And as someone that could make Mitsumi smile.
Sometimes he hated Hareta. He hated how his charms and charisma would be ignored, but anything Hareta did would automatically garner her attention. (Which, he didn't blame Hareta by a lot, half the stuff he did would get anyone to stare at him) She never smiled for him, but she always did for Hareta. It was a rather uncomfortable feeling, being jealous of someone like Hareta of all people. But he likes Mitsumi's smile, so he's happy even if it's not him that can make her smile, but he'll keep trying and waiting for the day when she turns and smiles at him, doing everything he can to impress her in the meantime until that day comes. He will do anything for the sake of her happiness.
Jun sometimes thinks about her eyes. He likes how bright they are. He thinks they're the eyes of a beautiful, intelligent, confident person. But at the same time, there's a clouded feel to them, as if hiding some kind of secret that he feels he has to know. He doesn't necessarily like it, but if he could find out just what this secret was that she was holding deep down inside her, maybe he could help make the clouded feel go away so that her eyes can continue to sparkle like they do when under the stage lights.
Jun might think about her body. Mitsumi definitely has a great figure, and it's a battle of willpower not to let his eyes wander south (a battle he usually ends up losing about seventy percent of the time, because hello, how could you not ignore the curves?) and he continues to shower her with compliments, even if it often ended up with his face meeting the dirt. (Girls were strange-weren't you always supposed to tell a girl that you thought they looked great?) Her great figure is probably one of the reasons why he feels the need to hug her, not as an excuse to feel her up or anything like that (Because Jun was a gentleman and would never try to run bases like that) but just so that he can feel her soft if not slightly cool skin and the way her hair smells like lavender.
He finds he can't look at other girls anymore without thinking about Mitsumi, without thinking, "Sure, that girl's pretty, but she'll never be as great as Mitsumi". It's as if she's become his princess, no, more like queen, from where she stands high above every other girl he's ever seen to unattainable levels of perfection. He can't explain why he likes her so much and what makes her so special, but when he sees her, Jun thinks about a happily ever after with a house and picket fence. When he's with her, he wonders vaguely what their future kids might look like. He imagines that they'd share their great looks, his sparkling personality, and have eyes like hers.
Jun wants to be the person that Mitsumi relies on, to call him her hero. He wants to be the one that finds Dialga so that he can show Mitsumi how great he is. He will take a bullet for her, shield her from any danger if it meant keeping her safe. The one that stands tall and proud, standing protectively over her as he faces down Team Galactic and declares, "The hero always arrives at the nick of time!", Staraptor waiting on command to blow the bad guys away.
However, Jun's not an idiot. He knows that he may not always be her hero. When he is beaten, watched as she is taken away by Team Galactic, he knows he's a failure. Because the hero never lets the bad guys get away with kidnapping the princess, his queen. He doesn't deserve to be the one whose name she calls to save her-even now he's sure that it was never his name, but Hareta, his rival in Pokemon and unintentionally in love, that she was always thinking of from where she was being kept in their headquarters. Even when he finds out her past, he knows it doesn't change how he feels about her, and she could be the world's most powerful, evillest super-villain and he would still love her, even as he would try to convert her to good or walk faithfully by her side as a henchmen if he had to. He only regrets that it wasn't him that could bring her back from the depths, but Hareta. But if it worked and made her good again, then so be it.
Maybe he is blinded by love, maybe he isn't, but he knows Mitsumi is his precious person, even if she doesn't feel the same way. He'll always be there for her, riding in on the back of his Dragonite as he holds a hand out to take hers, and he'll always be there to fly her away from all of her troubles if she ever asks him to. If she chooses Hareta over him, then he will give his blessing, because what matters is her happiness, not his. But that will never stop him from trying, to do everything in his power to show her just how great of a person he is and that what he feels for her is genuine. He'll say "I love you" as many times as it takes, with as much feeling as one can pack into those three little words.
So he'll continue to chase after her, continue to walk by her side from a distance, continue to try to get the girl because that's how the adventure always ends for the hero.
Even if there is a chance that, while she may always be his queen, he might never become her king.