Sienna is a new student that Alex has befriended at school. She is cooler than the people Alex would usually hang out with and therefore Alex is desperately trying to "cement the friendship" before Sienna finds out that she's a full social class above Alex.
In A Fair to Remember, Luke had a crush on Sienna and tries to get close to her by asking her and Alex to help him with the girl he likes. He gets Alex locked in the fake county jail at the fair and almost manages to kiss Sienna before Alex stops it. Luke then spills to Sienna that Alex is trying to hide that she's less cool than her.