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 tried to get close to her by asking her and Alex to help him with the girl he liked. He got Alex locked in the fake county jail at the fair and almost managed to kiss Sienna before Alex stopped it. Luke then spilled to Sienna that Alex was trying to hide that she was less cool than her.