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I was completely lost until you mentioned monsters in the title. While this most likely isn't the answer, there was a sports-themed episode I remember from the short-lived Playhouse Disney (now Disney Junior) cartoon Happy Monster Band.
If that's not the right show, then you'll need to give more elaboration on the details (animation style, TV network, voices, character designs, anthropomorphic species or humans, etc).
Edited by TVGuy2001No, that's not it. I remember precious little, unfortunately, but I know the male and female leads were human. I don't think it was an anime, but it definitely felt like it had anime stylings. I want to say the character designs were somewhat like Dragon Quest characters (or Dragon Ball, but slightly toned down and more 'fantasy' than 'martial arts'). I also saw it no later than mid-2000s. Pretty sure it aired either on YTV or Cartoon Network Canada.
Edit: The plot, I feel, was akin to that of Kirby: Right Back at Ya, but I'm not prepared to state that with any degree of confidence.
Edit 2: Thinking about it, Xiaolin Showdown might have been closer to what episodes were like.
Edit 3: Having gone through Wikipedia's list of shows aired for both and failing to trigger any memories, there's a small chance the cartoon aired as late as early 2010s. I'm almost certain it didn't, but I'm really bad with time. I feel like I was younger when I saw it, though.
Edited by VideocrazyI don't remember if there was a baseball episode, but could it be Nerds and Monsters?
Edit: Having just written that, it occurred to me, have you checked the Baseball Episode trope?
Edited by foxleyI had, but I checked again to no avail. Then I remembered the villains could probably have qualified for Goldfish Poop Gang and Terrible Trio, so I checked those too. The former had nada, but as I was looking through examples of the latter the page image immediately twigged my memory. What I was looking for was an anime, not a cartoon as I thought.
The show I was looking for was Mon Colle Knights, and wow I forgot a lot about it. From my original post, the male character was Mondo, the female character was Rokuna, and the mascot was Lovestar. I remember the three main antagonists too, but none of the supporting cast.
Anyways, thanks, I can cross another off my 'tormenting me for literal years' list.
Edited by Videocrazy

I remember one episode where the heroes play baseball against the villains for... some reason. One scene sticks out in my mind. The mascot is up to bat. The villain pitcher asks how the heck they're supposed to throw the ball across the strike zone (the mascot is about the size, maybe a bit smaller, than the batting helmet). The announcer and commentator cheerfully announce that they can't; they'll have to roll the pitch across.
I keep wanting to call the series 'Monster Hunter', but that turns up nothing by that name not related to the franchise.
Edit: The show in question was an anime called Mon Colle Knights.
Edited by Videocrazy