I didn't recognise her (Amber's) name but I really liked her character on the traveling pants films so looking forward to her on the show.
To my knowledge, Thirteen taking time off wasn't explained - I thought it was maybe that she was developing symptoms of Huntington's. Just something she said to Taub about how she thought she was ok but turns out she wasn't.. or something along those lines.
Peter: So how many are there? Is it bad? Olivia: Did you eat? Peter: Yeah. Olivia: Well, that's unfortunate.