Now, I'm gonna tackle both the original game and PMD so let's get the show on the road!

Trainer

Now when it comes to trainer oc's you need to give them a good looking style, a team that is fair, and a original name.

And if your character's rival is Gary, then please explain which universe your oc is (ex: ash doesn't exist in the universe)

Your pokemon team must be believable that you caught them. No Legendaries! (Unless if it's believable that you caught them like celebe, shaymin, and Victini)

I'll give you a comparison of my oc and a mary sue:

My oc:

Name: Kathrine Rogers

Age: 14

Personality: Bubbly, kind, brave, determined, enthusiastic

Looks: blue eyes, long orange hair tied to two long pigtails that reach her hips, a light pink shirt with a pokeball on it, black skirt, black leggings, red sneakers

Rival: Henry (another oc)

Team: Flame the charmander, Scratch the pidgey, silky the butterfree, peena the pikachu, and dusty the marowak

Strengths: If she is winning a fight, when everything is going okay, being with her pokemon

Weakness: cookies (she loves them), any mentions of her mother (unless if it's good news like someone knows where she is), loosing one of her pokemon

Flaws: she is very clumsy at times, she doesn't know everything about pokemon so she makes a mistake or two, she is a cry baby but she tries to hold in the tears

Mary sue:

Name: ASHLEY KETCHUM

Age: 16 (but she looks 10)

Personality: PFF WHO NEEDS EMOTIONS WHEN YOU ARE SO ATTRACTIVE >:3

Looks: One red eye on the left and one purple eye on the right, a tank top, HUGE CHEST, mini skirt, a kawaii neko ears and tail!

Rival: MISTY AND EVERY SINGLE GURL WHO KNOWS ASH SENPAI

Team: NEKO THE SYLVEON, ARCEUS, LATIAS, A SHINY EVEE, SHINY GARDEVIOR

Strengths: EVERY THING SHE IS IMMORTAL!

Weakness: NONE LOL IM SO PERFECT!

Flaws: WHO NEEDS FLAWS WHEN YOUR IMMORTAL!

PMD

Now, I don't care if your oc is a sylveon, evee, or some pretty pokemon as long as you are not a legendary.

It really bugs me when a oc is a legendary for some reason. Anyways I suggest when making a oc you take the little quiz at the beginning at the game.

The reason why I choose a charmander as my oc is because the quiz made me to one and I say that the oc is fitting to me.

I will give you a example with a good oc and a mary sue again:

Good oc:

Name: Maria

Species: ralts

Personality: quiet, shy, cowardly, friendly

Items: heal ribbon

Team: Hikko the snivy and Nii the vulpix

Strengths: Encouragment, being with her friends

Weakness: being alone, fighting stronger pokemon

Flaws: Even though she's the leader she isn't so tough, she's a doormat

Mary sue:

Name: SUGAR NEKO

Species: evee (BUT A SHINY)

Personality: PERFECT AND SMART

Items: GOLD RIBBON (even though they are completely useless)

Team: Mary the skitty and sueki the latias (YEAH LATIAS IS IN HER TEAM I KNOW RIGHT!?)

Strengths: EVERYTHING

Weakness: PFF SHE'S PERFECT

Flaws: IS THIS A JOKE SHE IS SO PERFECT

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Well I got nothing else to say, but make sure that your oc is original as possible!