Rudolf, the king of Ruritania is contrasted with Rudolf Rassendyll, the protagonist of the novel. Furthermore, Rassendyll can also be contrasted with Rupert Hantzau. 'Masterplots' (accessed by eNotes) by providing evidences from the text shows how Rassendyl has been presented as a foil. Rudolf "reinforces Rassendyll’s worst qualities even as he illustrates, by contrast, the best. On the other hand, Hentzau appears at a glance to be thoroughly different from Rassendyll, yet Rudolf’s fascination with Rupert’s attractive evil clearly suggests an affinity between them.” Thus Rassendyll has been presented as a unique character in the novel who can be compared and contrasted to two other characters of the narrative. The reference link given below provides further details.