Bertram

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The Griffin children's half-brother and Stewie's archenemy, Bertram first appeared in Emission Impossible as an unborn sperm. Though he and Stewie resolved their differences after a microscopic encounter, their hatred was rekindled in Sibling Rivalry, after Peter replaced all the sperm at a sperm bank with his own and Bertram's sperm went to a lesbian couple (One of whom was the gym teacher at Meg's school). Battling for control of the playground, Stewie and Bertram (along with other children on both sides) fought it out in the air, on the ground as swordsmen, and Stewie even briefly transformed into a Rambo-influenced commando. Bertram is a dishonorable opponent, as he used biological weapons (chickenpox) against Stewie. Stewie won the war, and Bertram admitted defeat and ran away.

Bertram shares many qualities of Stewie, these appear to include intelligence, strategy and rampant megalomania. Bertram also has a football / rugby ball shaped head like Stewie.

However, Bertram would make a comeback to face Stewie in the Family Guy: The Video Game; he is the last boss that Stewie fights.