Curt Lemon and Ted Lavender are basically the red shirts of the book. What, you never saw Star Trek? Fine: They're marked for death from their very first mention. Ted Lavender is our very first body to hit the floor, and while he continues to haunt O'Brien throughout the book, his main characteristic is that he is scared. He takes tranquilizers. And the guy who's scared to die is the guy who's first to die.
Lemon, while he's Rat's best friend, is essentially a more moral version of Azar, and therefore doesn't really need to stick around for the whole book. O'Brien uses him for "The Dentist" because using Azar there would make Azar too sympathetic too early. Other than that, he's unfortunately useless as a living character in this story, but very useful as a dead one—as dead people in need of memory, he and Lavender remain a part of the book until the end.