At first, Liesel and Rudy are just mere acquaintances, neighbors at the most. They live down the street from each other, and occasionally cross paths at school and in the streets, especially playing soccer and other games outside. But then, when Liesel takes down the neighborhood bully, a true friendship is formed. For Rudy, it is love at first sight. He is "wow'ed" by Liesel and her strength, and the two of them become friends and start hanging out together quite a bit. At first it is just occasionally, but after a while, they end up doing almost everything together, including thieving for local gangs, and stealing books. When they aren't at their individual homes, they are together. They are hungry together, tired together, in trouble together, stealing together, scheming together, being happy and carefree together, and even rebelling against the local service requirements together.
For Rudy, it's more than friendship; given their pretty young age though, it stays friendly, with Rudy teasing Liesel about getting a kiss. Liesel's feelings for Rudy are more ambiguous; we know that she cares for him, but it isn't until later in the novel when tragedy occurs, that we realize to what extent. Their affection for each other is strong; they would do anything for each other. Rudy even jumped into a frozen river to snatch Liesel's book for her, risking freezing to death. They have each other's backs and are the best of friends with a very strong bond. It's a great relationship, one that grabs the reader and helps them to want to keep reading right on through to the end. I hope that those thought helped; good luck!