No, you cannot steal other trainers' Pokemon. If you try to (you throw a Pokeball at the Pokemon), the trainer will block it. If you want what they have, you just have to raise your own!
you hack with action replay
You can cath alot of Pokemon That helps... But really, there are many of them. all of them can be caught. except other trainers. but with a gameshark, you can catch all Pokemon you meet. but other trainers' Pokemon become bad eggs.
If you want to you can try the other modes like single or knockout. Not your thing? Train more Pokemon and get all the Pokemon in the pokedex. And meet some other Pokemon leafgreen and Pokemon firered players.
Yes there are other trainers in game, commonly known as non-playing characters.
you can't. :(
Yes
they dont
Never in any game of Pokemon can you catch every Pokemon on one version of the game you must use the other versions to get the rest. Unless you cheat but don't its not worth it.
You train it by fighting other trainers or other wild pokemon.
You can't catch or obtain other trainers Pokemon in emerald.
You can use a Action Reply codes to catch other trainers pokemon.
You can't catch other Trainer's Pokemon. It is not an intentional feature in the website.
You can't steal Pokemon however in Pokemon colosseum and Pokemon xd gale of darkness you can snag shadow Pokemon from trainers.