Three years before the events of the novel, Ralph Marvell had a brief affection with Clare van Degen. It ended poorly, which caused a change in Ralph's romantic life, as described in Chapter 6:
Ralph, since then, had reached the point of deciding that he would never marry; reached it not suddenly or dramatically, but with such sober advisedness as is urged on those about to take the opposite step. What he most wanted, now that the first flutter of being was over, was to learn and to do––to know what the great people had thought, think about their thinking, and then launch his own boat: write some good verse if possible; if not, then critical prose.
This description of Ralph's approach to romance shows how his character is a foil to Undine's. He is, in many ways, her opposite, and their differences serve to make each character more distinct. Chapter 6 is a long biography of Ralph Marvell, describing his past and his character. Throughout the chapter, Ralph and Undine are contrasted. While Ralph hardly ever advocates for his own self-interest, Undine's first priority is always getting what she wants. Ralph prefers reading, contemplation, and (sometimes) writing, as opposed to love and relationships, whereas Undine does not read and gets bored with Ralph's literary talk. Their most stark difference, as in the passage above, is that Ralph is slow and careful in matters of love, often overly thoughtful to the point of inaction, whereas Undine's affections change often and rapidly. Ralph's characteristics contrast with Undine's and highlight the strength of her ambitions and desires.
But notably, this does not make Ralph any more likable or honorable as a character. Ralph is by no means a foil to Undine because of some moral perfection that shows her flaws. Ralph is just as vapid as Undine, if only toward his own work and writing. Ralph and Undine are both flawed, but their differences, especially in matters of love, serve to contrast and elaborate each other's character.