Kitty Song-Covey is all grown up in ‘XO, Kitty!’ Anna Cathcart opens up to ET’s Deidre Behar about Kitty’s evolution, from 13-year-old little sister in ‘To All the Boys I’ve Loved Before,’ to now playing the romantic lead. Anthony Keyvan, who plays Kitty’s on-screen BFF, reacts to being a part of the series and why