"Who Am I" was written and produced by Katy herself,Craig David,Jr. Blender,Diplo and produced by Geneeus,Blender and Diplo. It's a midtempo R&B song,with smooth vocals delivered by Brien and David. Katy later admitted:
"[This song] was about my first boyfriend. I used to be in a band with him and when we broke up... I realized everyone I knew in London was in the band. I felt like I'd have to give up my band,my friends,my whole identity,start again from scratch. You don't just lose a person in a break-up. Others might lose their home,even their child and family,I think of people going through divorces when I sing it,too. It was originally a solo song,and I still don't really see it as me singing to Craig,we're the same person singing the same song."
