A "care miss" is required on the Pix, the Smart, and most older tamagotchi, in order to raise a specific character. The care miss method is:

• Wait until they get fully hungry or fully unhappy (I find it harder to get the happiness to decrease, and prefer to do these steps with hunger. But you can let them get fully unhappy and follow the steps below, if you prefer.)
• They’ll cry out your name. Wait 15 mins.
• After the 15 minutes are up they’ll either get sick, or nothing happens. Either is fine, you still got the care miss. If they got sick you can heal them now. You can also feed; if you need multiple care misses, just feed them ONCE.
• For multiple care misses they’ll be fully hungry again in one hour, and the process will repeat. Do this as many times as indicated to get your specific character.

When I do this, I keep a tally of care misses! I also run a timer once they call out my name, so I know for sure it’s been 15 minutes.

Note that on the Uni, if you feed the tama you have to wait an additional hour for the next Care Miss. So you may find it easier with the Uni to not feed the tama after the 15 mins are up. Just make sure to heal any sickness, and then wait for the next hour to pass without feeding.

This is also the best method for the Tamagotchi Friends and TamaGo.