Out of all the tama characters - Ojitchi and Otokitchi are the only characters you can get for certain just by making your tama get old- just don't mate with another tama and keep refusing the matchmaker and after 6 days(unpaused) of your tama being an adult- (around age 10) you will end up with an Ojitchi if you have a boy, or a Otoklitchi if you have a girl.
Make sure, if you do this and you wish to carry on your generations, that you have access to another tamagotchi to mate with as Ojitchi and Otokitchi can ONLY mate with each other. Then you will have Oyajitchi's evolve from their babies.
I don't think there is a definite way to get any other character.