I can explain the first question with pretty good detail. He's a kid himself, but in the Millenium Puzzle around his neck is contained the spirit of an ancient pharoah. In duels, he allows the pharoah to play for him because the pharoah(named Yami) was alive during the time that the mosters were real in Egypt, therefore he is more experienced. The fact that it looks like he's older is because Yami happens to look a lot like Yugi.
to your second comment, you'll have to earn Eevee's trust. I'm the only 1 she ever trusted on first sight. Why I don't know, but when we(latios, latias, and I) found her, we got her out of a trap she was caught in and she would only let me be the 1 to bandage her leg. she'd try to crawl away with her 3 good legs when Latios or Latias came close, but she'd just sit there when it was me. She came to trust them in time, but you still need to earn her trust.

