Stewie is the youngest of the family.  He has aspirations of world domination and views Lois as his great adversary.  Stewie is always working on inventions whether it be a mind control device of a laser of death. 

Despite his intelligence, the entire family still treats Stewie as a child, a fact that he loathes.  He is constantly trying to kill Lois though all his attempts have thus far, failed.  Stewie's child side is still very present in his life (he often talks to Rupert, his teddy bear, or pretends that his Sesame Street phone really works).