You start off with one character, but when you get married the spouse stays with you, and so does the baby (that's why I have 3 characters on screen). They leave after a while, though.
Last but not least, here's a growth chart. Hope I helped, and sorry if that was confusing in any way. Ask me and I can clear it up.
Tama-Go is my favorite version! You should probably buy a few figures for it to fully appreciate it though. I didn't like mine until I bought two figures and then POOF! Instant favorite Tamagotchi