I wouldn't say that. If you pretend Kintarou is his own character, he acts pretty differently from Fuutarou in the one chapter he "shows up".
He doesn't tease Nino one bit, but he gives the impression of a delinquent since he smokes and he's scratched up. Instead of talking about math and language, he's talking about stars and charms. Apparently Fuutarou is scared of the supernatural, but we've never seen that until now so it's effectively Kintarou's character trait.
Even if he didn't save Nino, I can kind of see her being charmed by "a bad boy hiding his sensitive romantic side".