Well, that isn't the only way to get a secret character. Debugging lets you choose your character as well.
But anyway, a secret (rare) character is a character that is hard to get, or doesn't come around too often. Like binary said, Oyajitchi is counted as a rare character, and to get it, you must mate Ojitchi and Otokitchi... there's no other way to get Oyajitchi... he doesn't come on his own like a Kutchipatchi or a Tamatchi, ect. does. He isn't even like the other rare characters (Zatchi, for instance) who come just randomly.
-TG ^.^