What friendship between liesel and Rudy. How dose their friendship change and grow throughout the novel

They begin as neighbors, but Rudy definitely has a gigantic crush on Liesel....... alas, they're too young for anything more than crushes. Rudy regularly teases her for a kiss, and they become very close friends (the best of friends). Over time, the depend on each other and become inseperable; they are each of them a life line for the other.