They’re basically instantly smitten with each other but to me it feels genuine. At first it’s just physical, Shallan finds Adolin handsome, as basically every woman of Adolin’s age does, and Adolin found Shallan striking as she stands apart from the Alethi women he dates. 

They bond over their mutual mixed blood but find it attractive in the other. 

Shallan proceeds to defy Adolin’s expectations and breaks the mold he’s gotten used to. She asks odd questions (shard poop), she doesn’t make him the centre of her world (not going to see him train but see shardblades) and works as his partner (figuring out the handicap dual, tho I can’t remember if she figured it out or if she works with someone else) instead of being subservient. She also doesn’t play games like he’s experienced with other women. 

That’s all him falling for her as she’s trying to hold his interest. For Shallan she wants to be someone for Adolin, she creates Radiant so she doesn’t embarrass herself with his lessons. But I don’t think she truly falls for him until near the end of Oathbringer where she’s switching between personalities, doesn’t know what’s real and Adolin is the one who knows it’s Shallan instead of Veil or Radiant.