Katherine "Katie" Gallagher (previously Maguire) is the daughter of Phillip Gallagher and Mandy Maguire. She was born during Series 3 when a pregnant Mandy was knocked over during a robbery at the Karib's Shop. Katie is named after her maternal grandmother, Mimi Maguire.
Katie returns in Series 11 with Lip after Frank wants to see her. Frank finds out that Katie and Lip lived close to the Chatsworth Estate and Katie has been visiting her grandmother Mimi secretly. Katie's age has been changed to 9 years old despite being born in 2006. Living with her father nearly all her life, Katie has learned to dress and act like a boy, which Mimi doesn't approve of, but Frank, whom Katie has formed a bond with, tells Katie that 'Gallagher's do it their own way'.
- Katie was originally credited as "Katie Maguire" however upon her return in Series 11 she became "Katie Gallagher". This may indicate that Lip changed her last name to reflect his, something that the Maguires would not have allowed when Katie was born.