Where this began
In the Shadow of Young Girls in Flower (translated by James Grieve), pp. 359-369.
Bloch mystifies the narrator by his effect on Françoise, who seems to have expected some "prodigy of nature" and is disappointed when she meets him: "She seemed to bear me a grudge, as though I had misled her about him, or exaggerated his importance." She's also disappointed when she finds out that Saint-Loup, "whom she adored, ... was a Republican." But Françoise, who is a royalist, gets over it, "and when she spoke of Saint-Loup, she would say, 'He's just a hypocrite,' her broad, kindly smile showing that she thought a well of him as before and that she had forgiven him."
We learn more about Saint-Loup and his mistress, whom the narrator credits with a positive effect on him. His family "did not understand that, for many young men in fashionable society, who might otherwise never acquire a certain cultivation of mind or a measure of mildness in friendship, who might never be exposed to good taste or gentler ways of doing things, it is often in a mistress that they find their best teacher, and in relationships with such women that they make their only acquaintance with morality, serve an apprenticeship in higher culture, and learn to see the value of knowledge for its own sake." (Imagine an English or American writer contemporaneous with Proust making such an assertion.)
An actress of sorts, the mistress made Saint-Loup "see the company of fashionable ladies as insipid and the requirement to attend their functions as intolerable." She thereby "saved him from snobbery and cured him of frivolity." But things do not go well between them. Her friends, writers and actors, make fun of him, and she asserts that "their worlds were too dissimilar." There's also an implication that she is gold-digging, and that "she would wait quietly until she had 'made her pile,' which, in view of the sums doled out by Saint-Loup, looked as thought it might take a very short time." His ill-advised suggestion that she perform a scene from an avant-garde symbolist play for guests of his aunt is also a disaster.
And then the narrator goes into a fit of jealous pique because Saint-Loup asks his grandmother if he can photograph her before he leaves Balbec. His grandmother and Françoise make so much fuss over the request that the narrator gets huffy and the grandmother takes offense at his attitude. He retreats into childishness again.