A look-alike is a person who closely resembles another person. Appearance may be the only connection between the two people, as they may not be related, or their personalities may be quite different.
- Thaddeus and Thor and Phineas and Ferb — similar in appearance, but opposite in personalities. ("Thaddeus and Thor")
- Jeremy and Charles Pipping IV — similar in appearance. ("Flop Starz", "A Hard Day's Knight")
- Ferb and Ferbgor — distant relatives nearly identical in appearance. ("Rollercoaster", "The Monster of Phineas-n-Ferbenstein").
- Isabella and the Unnamed Victorian age Isabella lookalike — Similar to attitude and sweetness and same to say the line "Whatcha' doin?" ("Rollercoaster", "The Monster of Phineas-n-Ferbenstein")
- Candace and Constance — They both want to bust their relative.
- Phineas and Dr. Phineastein — Have the same optimism and attitude, although Dr. Phineastein has a more evil personality.
- Jeremy and Jeremiah — They both look alike, and Jeremy likes Candace in the same way Jeremiah likes Constance.
- Heinz Doofenshmirtz and Jekyll Doofenshmirtz — They are not only related, but are also alike in attitude, always adding something to their inventions, such as a self-destruct button, or a fairy princess dial. They also aspire to be evil, but usually fail.
- Roger Doofenshmirtz and Jameson — They appear to be related since Jameson may be the ancestor of Roger, execpt for the fact that Jameson worked as a butler for Jekyll, who is an ancestor of Heinz, who is the brother of Roger. They also look alike.
- Perry the Platypus and Unnamed robe-wearing Platypus — They are both anthropomorphic platypuses.
- Perry the Platypus and Platypus monster — They make almost the same sound and fought against their nemesis.
- Betty Jo and Lorraine —They are identical twins.