Katie was someone who was close to you, the closest to you and wanted to keep it that way. She was overprotective, clingy and possessive. She was willing to kill or do whatever it takes to keep you safe and around her but the others in TSP don't care about as long as it doesn't get in the way.

Katie: "Where were you? It's late. The infecteds get extremely aggressive at night. What if you got hurt or worse?"