Well, I'm not sure but I got a memetchi for my first tamagotchi today. Basically take good care of it, you can let happiness drop down to 2 hearts, but I never let hunger drop more than one heart. Sometimes I accidently put my tamagotchi in time out on accident (cause it didn't play with it needed a heart for happiness but it just needed to eat first....which also explains why i had happiness down to two hearts, cause i accidently timed it out when it didn't need to). Also I got full training bars. I kept my tamagotchi's weight 1-2 lbs above or exactly on the minimum weight.
Hope that helps.