Yami
Now here's where things get complicated. Yami literally means dark. Yugi calls him that because... well, he can't remember his name! He says "I have been called many things, I have been called Yugioh, pharaoh, I have been called Yami" (or something like that) and Yugi chooses to call him Yami. (I've read that in the memory arc we learn his true name is Atem or Atemu) So anyway, he's the 5000 year old spirit who lives inside the puzzle (and now inside of Yugi). If you're wondering what the puzzle is: Yugi's grandfather gave him a puzzle to solve, it formed a pyramid and turned out to be one of the seven millennium items. That's all I'm gonna say here, but if you want to know more look under 'Millennium Items' in the miscellaneous section. 5000 years ago Yami was the Pharaoh of Egypt. He saved the world from a great evil (and probably from being destroyed). Trouble is, that's about all he remembers about his past. He's a little confused, searching for answers because he knows the past in repeating itself. But he's a good guy: noble, just, courageous, he can seem a little unfeeling but that's just his ancient sense of justice. Overall he's really nice, and can be very kind. He looks out for the innocent and for his friends.
Anyway. Yami inhabits Yugi's mind, but only when Yugi is wearing the puzzle. He can take control of Yugi's body (changing Yugi's appearance and voice in the anime) He does this whenever Yugi can't handle something (like when he faces up to a bully with a knife or when he's in a really important duel). Yami is an expert duelist because Duel Monsters originated in ancient Egypt. In fact, he earns the title of King of Games when he defeats Pegasus. No matter which of them is controlling Yugi's body the two can communicate with one another. And they can meet 'face to face' inside their 'soul rooms', a place created in Yugi's mind by the power of the puzzle.
Age: Well, about 5000 + twenty some odd would be my guess (sure on the 5000 part though, they say it a lot!)
Millennium Item: Millennium Puzzle
Hikari: Yugi