In his debut, "Emission Impossible", Bertram was still a sperm. He entered combat with Stewie Griffin, the youngest son, to prevent his destruction, as Stewie still wanted to be the baby of the family. Here, the two became frenemies, and Stewie would change his mind on a new family member because of this.
In "Sibling Rivalry", a mishap prior to Peter's vasectomy landed Bertram in a vial that would later be used to impregnate the gym teacher's partner. After he was born, he started playing at the same park as Stewie, and the two fought over who would be ruler of the playground. Stewie eventually wins by disarming Bertram, and it appears that Stewie killed Bertram, but in reality was only planting a tree. Bertram actually admitted defeat and ran away .
He appears again in "The Big Bang Theory" where he tries to erase Stewie from the universe by killing Leonardo da Vinci, one of Stewie's ancestors. Although he succeeds in killing da Vinci, he is killed in turn by Stewie when he is shot in the head with his crossbow. In the end, Stewie manages to maintain the time line and overall save the universe.
Bertram shares many qualities of Stewie, these appear to include intelligence, strategy and rampant megalomania. Bertram also has an American football/rugby ball-shaped head like Stewie.
He appears in "Family Guy Video Game!" as the last boss that Stewie fights. An alternate reality version of Bertram appears in Family Guy: Back to the Multiverse as the main antagonist, seeking revenge on Stewie and Brian. He attempts to build an army using multiple alternate dimensions and their occupants, but is foiled by Stewie and Brian's efforts. In the end, he attempts to kill them with an army of clones and a weaponized Tyrannosaurus rex, only to be defeated once more. He is then finished off when he is fed to his dying dinosaur.
Bertram was voiced by Wallace Shawn, however Scott Menville did his sound effects.