If you treat your Tamagotchi the same each time, you have more chance of getting the same character(s). Tamagotchi characters are choosen by the way you treat it. This means how many training bars, how fast food/happines indicators are filled etc..
Try treating it differently.
Note: If you are going to treat it worse than what you normally do, make sure you do not kill it.
If you treat itthe same each time, it will turn into the same thing each time. Try to feed it more snacks and clean up the poop later than you usually do. ^_^