Liam is the son of Heinz Doofenshmirtz, but not featured in the episodes since he was in school at the time because he is 11 years old. He likes being evil and always follows his dads inventions so concidentlly, he knows Phineas and Ferb. But that also means Liam is the only one that knows his dads inventions always make Phineas and Ferbs inventions disappear. He would tell them, but that might lead to his dad being arrested and he would'nt want that to happen, would he? He also knows that Ferb loves his sister, Vanessa but everyone knows that now.