Getting different characters on both the iD L and the 4U is the same; it's determined by the number of 'care misses'. That is, not responding when your Tamagotchi shouts at you because it's hunger/happiness meter is depleted. Once the attention icon is no longer lit after the Tamagotchi bleeps, if you have not fed/played with it, then you have a 'care miss'.
You can find growth charts here (iD L) and here (4U).
To train on the 4U you take them to school or by praising certain actions when they are a toddler; more information can be found here.