Rudy is Liesel's first friend on Himmler Street and remains her closest friend throughout their childhood. Though he is seen as "a little crazy" by their neighbors, he and Liesel form a fierce bond and protective instinct for each other after they meet at a soccer game, when Liesel is the first to successfully keep Rudy from scoring on his penalty kick. From this point on, their relationship is marked by intense competition. They are constantly daring each other into dangerous acts or placing bets on who can outdo the other.
At the beginning of Rudy and Liesel's friendship, their lives revolve around each other. They are schoolmates, playmates, neighbors, and best friends. However, as they grow up, their lives begin to diverge. Though they continue to spend time together, often stealing food and supplies to feed themselves, there are things they no longer share with each other. Rudy is enlisted in the Hitler Youth, an organization solely for boys, the details of which he doesn't always reveal to Liesel, and Liesel hides the existence of Max Vanderman from Rudy. Their friendship is also complicated by Rudy's growing feelings for Liesel.
In their first bet over who is fastest at a hundred meter dash, Rudy asks Liesel for a kiss if he wins. However, the race is a draw.
Rudy looked over, all sharp teeth and gangly blue eyes. His face was painted with mud. "If it's a draw, do I still get my kiss?"
"Not in a million years." Liesel stood up and flicked some mud off her jacket.
"I'll get you out of goalie."
"Stick your goalie."
As they walked back to Himmel Street, Rudy forewarned her. "One day, Liesel," he said, "you'll be dying to kiss me."
Throughout their childhood, Rudy frequently asks Liesel for a kiss and is denied. At first, it starts a playful game, but as they get older it is clear that Rudy's feelings for Liesel are stronger. He often puts himself at risk to help her, such as by helping her steal books or by jumping into a freezing river to rescue one of her books. Though books are not important to him, he recognizes their importance to Liesel and risks arrest and even freezing to death in order to obtain them.
Though Liesel cares deeply for Rudy, she never reveals her affection to him. She spends most of their childhood convinced that she both hates and loves him. When she finally shows Rudy The Word Shaker and tells him about Max, she realizes how much she cares for him.
At first, Liesel could not talk. Perhaps it was the sudden bumpiness of love she felt for him. Or had she always loved him? It's likely. Restricted as she was from speaking, she wanted him to kiss her. She wanted him to drag her hand across and pull her over. It didn't matter where. Her mouth, her neck, her cheek. Her skin was empty for it, waiting.
Unfortunately, her realization only comes...
a month before the bombing of Himmel Street. When Liesel finds Rudy's body in the debris, she shakes him and confesses that she loves him, begging him to wake up. When he does not, she finally grants him the kiss she always denied him in life.