She is Jeremy's little sister.
Phineas and Ferb: Forever Summer
She and her friends and family are older and the setting is in the future in this series. Since her brother is married to Candace, Suzy is now her sister-in-law.
She is a minor antoginist in this show. She appears in Epsiode 3, where she is mean at beginning then good, but then she says "It's so hard being nice *alarm rings* Oh! yay time to be mean again".
She appears in the television special, Jenny With a Chance: Christmas Special.
She does not appear in season 2.
She made a cameo in the series, but is often mentioned as series progressed.
Jeremy Johnson - He is Suzy's older brother.
Candace Flynn - She is Jeremy's girlfriend and Suzy's nemesis.
Starscream - He is a Decepticon. He wants to help Suzy destroy Candace to achieve his goal over his leadership of the faction. He is Suzy's best friend.
Jimmy Nelson - He is Suzy's crush, She respects him because he will not let anyone date his sister much like she will not let anyone date Jeremy.
Cain Rangle Johnson - He is her older cousin, but likes Candace, he knows that Suzy is a "devil".
Optimus Prime - He is the leader of the Autobots. Once, the Autobots received Starscream's signal. Then Optimus tracked him down to Suzy's house. Suzy found out about this and ordered Starscream to injure Optimus Prime. He did, and it went successful. Prime was so badly damaged that Ratchet had to work overnight repairing Optimus.
Wilson, character from Dark Adventures greatly despise Suzy than anyone else, even more so than Candace Flynn despite never interact each other. He despised her as he saw her harm Candace every time Candace dates Jeremy once Jeremy not around once.
Phineas Flynn - Phineas is unaware that Suzy's dark personnality, but is good friends with her. He and Isabella gets very annoyed when Suzy copys their cathphrases. In Candace and Jeremy's wedding, when Suzy hears that Jeremy and Candace got married and that Phineas & Isabella planned the wedding, she was furious and plans a attack! She goes over to Phineas & Isabella's backyard where she beats them with a bat as revenge! Phineas & Isabella were badly injured and had to be taken to hospital.
Stewie Griffin - Stewie & Suzy become really good friends once they meet in Phineas & Ferb Meets Family Guy. They love to pull pranks on Candace, and Suzy's amazed of Stewie's vocabulary & hatred of his mother. Stewie's & Suzy's behaviors are similar to Suzy with Candace when Jeremy's around. When Suzy discovers that Stewie nearly tortured Candace to death, she's immedeately pissed off and stops being friends with Stewie, freeing Candace and treating her wounds, then starts pranking her again when Jeremy arrived.
Kenny McCormick - Not much is known about Suzy's relationship with Kenny other than the fact that she killed him once in Phineas & Ferb go to South Park, because Suzy mistook Kenny for Candace.
Bart Simpson - Bart & Suzy grow a friendly bond when they met in Phineas & Ferb Meets the Simpsons. Just like with Stewie, Bart & Suzy love pulling pranks on Candace. Suzy loves pulling pranks as much as Bart does, and she is not afraid of bullies like Jimbo, Kearney, Dolph & Nelson. At one point, Suzy even copies Bart's catchphrase "Don't have a cow, man!", when she hurt Candace after Jeremy arrived.
Brendan Townsend - TBA
In the future
During the events of "Future Danville", it's revealed that Suzy has given up trying to cause trouble to Candace and instead works as a nurse at the hospital. She has become very polite and learned to admit that Jeremy will hang out with any girl he likes and that there's nothing she could do about it.