He's a endearing little guy who undergoes great development over the course of the game. He's also one of those video game characters that I can relate to the most.
Final Fantasy IX in general has an amazing cast of characters, but I found Vivi to be my favorite out of all of them.
Seriously, Raziel only wins by a hair in my book and it's mostly because Soul Reaver was the first game I played in the series. So if it wasn't for nostalgia, Kain would be the winner.