Liesel does kiss Rudy, but only after he is dead. Rudy's quest to get a kiss from Liesel is a recurring element in the book. It begins early on, when Rudy challenges Liesel to a foot race. Rudy agrees to let Liesel play a different position than goalie in soccer if Liesel wins, but says that Liesel must kiss him if he wins. The race is a tie, and when Rudy asks if Liesel will kiss him anyway, she refuses and makes a vow with herself that she will never kiss Rudy.
This is a vow she manages to keep for most of the book, even though she and Rudy grow closer as time goes on. Rudy is always doing little acts of service for her, and demanding his kiss as payment, and Liesel always refuses. It becomes a bit of a joke between the two. Liesel's continued refusal to kiss Rudy becomes an ironic indicator of their intimacy.
Rudy, along with everyone else on Himmel Street, is killed during an Allied bombing run. Only Liesel survives, because she hides in the basement working on her book. When Liesel sees Rudy's body, she finds words for what she has been feeling: she tells him that she loves him and kisses his dead lips.