Your pet cannot lose health, but you can. Wait for your health to return, before you enter another battle, if that is what you mean.
If you do not feed or visit your monster on Moshi Monsters, its health will go down and it will become sad and eventually will become ill.
There's not a set location for where Olimar loses Louie, this moment happens automatically right after you pay off all the debit. After that he will wander aimlessly through the planet until he ends up in last floor the Dream Den, where you find him.
you can't use him/her for the rest of the game as a combatant
It loses its charged.
it loses nutrients
It dies
It dies
she dies
it losews moisture
it loses its "magic"
It Eats Cheese