To get him is randomly. He/she was the first adult i ever had. Take good care of your tama. Make sure it has high displince. PlaY MILLONS of games with your tamagotchi and connect often. So there's no definte chance of getting Leaftchi. But there is a definte chance of getting Masktchi and Gozarutchi.
Here's how i got Leaftchi (chart)
Teletchi (girl)- Tamatchi- Young Mametchi- Leaftchi