Much like the original Vader, he seeks to rule the galaxy and wouldn't hesitate in slaughtering innocents to achieve this goal. He appeared in Blue Harvest, Something,Something, Something Darkside, and has recently appeared in It's a Trap! Darth Stewie later redeems himself by killing Emperor Carter, and accidentally has his neck snapped when his son Luke (Chris) tries to take his helmet off.
He later appears alongside with Yoda (Carl), Obi Wan Kenobi (Herbert) as fully redeemed and a force ghost, but he gets angry at Chris/Luke, extremely infuriated at the fact that he killed him even though he would've made it.
Darth Stewie is far more like Stewie Griffin than Darth Vader. He is laid back, fun loving, carefree, rather obnoxious, has a very camp sense of humor and is far more social and merciful than the actual Sith Lord. He converses casually with just about anyone, even if he's about to kill them, and is struggling on admitting to himself and everyone around him that he's a homosexual. Darth Stewie is capable of laughing with his enemies and does not take himself as seriously as Darth Vader did. He is an avid fan of Richard Mark's Hold onto the Night and seemingly still wears a diaper due to actually being one year old (despite having a teenage son), which is a fact he uses to tell Star Wars poo jokes.
He breaks the fourth wall on occasion and is notably aware that the Family Guy/Star Wars trilogy is a TV show but seems to just enjoy going along with it. He has a wicked side and, like Darth Vader, will kill almost anyone to fulfill his and Emperor Carter's goal. He also refers to being gay as "The Dark Side", showed no remorse at destroying Alderaan, killing his minions for trivial reasons, or cutting off his sons' hand. He also has very little if any patience for his exasperating Hispanic maid, disapproves of high gas prices and misses going out dancing. Darth Stewie does, however, respect good manners and saved Luke from Emperor Carter because he asked nicely.