Easy way to do this?

Well you know in a RPG when u walk in the back of a house you dont see ur char and when u walk infront part of ur image is over the house, or when u walk under a bridge u dont see ur char how would be the simpless way to draw a char under and image and when its infront on the image?

Dunno if you follow me.

Oh and how could I make like a invisible square under my char for collision?

Thanks in advance be really specific me dumb :P