There is six characteristics:
Happiness - happiness of your pet
Hunger - hungry pets sleep more and become irritable
Energy - spent while awake, spending increases during games or some activities
Playfulness - increases then nothing interesting happen, pet can follow mouse or call ball when too high, decreases then play
Irritation - increases then dragging or hungry, can lead to aggression, decreases then pet doing agressive things or by eating
Health - decrease means much more energy loss and much higher sleep time, health increases while pet sleep
there is some hidden options to increase or decrease stats

modifiers meanings: if more of them = faster this stat will increase and slower decrease, if less = faster stat will decrease slower increase
for example, if energy modifier higher then sleep time will be decreased (cause energy gaining by sleep will be increased), and energy loss will be decreased = sleep will be needed less often

This was 1/3 of pet behavior and individuality system, there is much more (such as preferables, actions and other), but understanding this part can provide creators information to help calibrate templates, this way they are more likely to provide experience they want