Help

Beat all of the levels up until the one where Lucas is being chased. Then, in that level, you will unlock Pokemon trainer.

We shouldn't have to write a walkthrough. The only way to learn how to be Pokemon trainer is through experience because you're mind memorizes how the character works. If you want experience, try training mode.
P.S.: It would also be hard to have to play and keep pausing the game so you could look at the walkthrough.

Well I don't know about walkthrough but here are just a few pointers

Squirtle: small target so he is hard to hit, he is agile so you might want to use him to rack up damage because he is quick, his recovery isnt great but he has great aerials so he is pretty good for damage racking

Ivysaur: he has a decen recovery but can be edgeguarded easy, he is great against aerial characters like Pit, Metaknight, ivysaur is an anti aerial fighter and he has projectile attacks his grabs are pretty good

Charizard: the powerhouse of the 3, I would use charizard to finish an opponent
Charaizard can glide but very slowly so he has a decent recovery
Charizard is extremely strong but slow so only use him to finish off opponents his flamethrower is a great edgeguarder


Hope this helps

Well just train yourself and than well you get the idea

I was having a hard time learning link in the first one and than after the second game I beacame a pro, than I had problems with zelda and when it came to brawl I mastered her too so give your self time to figure it out the character

In regular Brawls: Automatically unlocked
In Subspace Emissary: Help Lucas. Then you will start out with just Squirtle. Beat "The Ruins" stage to find Ivysaur and battle Charizard.