Once a firefighter, Leah Kerns now replaces Lewis "Lou" Young after he sacrifices his life to save his team members in the the 10th episode of season 2 "One Wrong Move." The only other woman to be working on Team One is Constable Julianna Callaghan. After Team One lost a valued member, Leah is called in to fill Lou's position. The first episode Leah Kerns appears in is the 11th episode of season 2 "Never Let You Down" the first episode after Lou is killed in the line of duty. Her presence is not well-received as the others are still trying to cope with the death of their teammate. She sympathizes with them, stating that she lost a friend from back when she was a firefighter. After her first Hot Call, Leah brings in armbands with Lou's name engraved on them, to remind them of their friend.
However, in the beginning Season 3 she seems to have been faded out. She's said to be on leave to deal with a family emergency. In Season 5, episode "Run To Me", Leah has returned to the team, having previously left to assist her parents after the Haiti earthquake.
After Leah joined, the female bathroom for Team 1 was changed from 'Jules' to 'Women' since there was now two females on the team.
Sam knowing what it was like to be the new kid, was the first to befriend Leah.