Eggsteps-like variable for Pokemon

Hi.

I'm experimenting lately with variables. I did a small exercise to create an eggsteps-like variable not for Eggs but for Pokemon which value would increase/decrease with each step in the overworld. After reaching a given value, a message would be displayed for each individual Pokemon in the party. Since this variable would be accounted at different times(depending on the Pokemon withdrawn/deposited with the use of PC, newly caught etc.), the messages would be then displayed at different times. Unfortunately, so far the value of the variable is treated globally for all of the Pokemon in the party and it doesn't matter when the Pokemon was put in the party - the messages pop out at the same exact time.

It may be a simple thing but I seem to be at a loss? Do you guys have some idea for creating one like this?

I tried to get some inspiration by going through Poison Effect, Happiness(Steps) and Eggs Hatching but it doesn't seem to give me any idea how to proceed.

Any help will be appreciated ;)