How has Rudy and Liesel's friendship developed from the beginning to Part6? Why is Liesel unable to return Rudy's affection and why she would only be able to when she sees his dead body.

Rudy and Liesel are best friends; he's her partner in crime, and he loves her. Liesel's feeling for Rudy are mixed, she isn't willing to take the step that would mark them as a couple because she's afraid of losing her friend. As gallant, heroic, and flirtatious as Rudy is, he won't get the kiss he waits for until after he's no longer able to appreciate it.

"As long as both she and Rudy lived, she would never kiss that miserable, filthy Saukerl."

And that's the way it was. Foreshadowing at its best.