The Champion of the Hoenn region in Pokemon Ruby and Sapphire is Steven Stone.
If you mean "Is Juan the leader in Pokemon ruby?", then no. It's Wallace who's the leader. Juan is leader only in Pokemon emerald. The champion is Wallace in Emerald, but Steven is the champion is Ruby and Sapphire.
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After becoming champion of Hoenn by defeating the pokemon league
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Wallace he was the former 8th gym leader in ruby and sapphire
on the top of sky pillar in Ruby and Sapphire you need to beat elite four and the champion
Juan is a gym leader in Sootlopolis City. To get there, you have to use dive. Wallace will be the champion. In ruby and sapphire, Wallace will be gym leader and Steven will be champion.
I'm going to assume you mean "Pokemon Ruby" in your question. Anyway, to trade between Ruby and XD, you must beat Pokemon XD and defeat the champion in Ruby.
It's because on Pokemon Ruby and Sapphire, Steven is the league champion and on Pokemon Emerald Wallace is the champion instead of the final Gym Leader. Juan doesn't exist on Ruby or Sapphire.
it is a random encounter after you beat the champion and the elite four, latios could appear anywhere
You can't, you can only get it in Ruby. But, you can get Groudon through certain events after you're the champion.
Do you mean the game or the male rival? Anyway, a Pokémon game is considered "beaten" when you beat the Champion.