It’s Carter Page. He was the subject of the FISA Warrant. He was a paid FBI informant/asset in a previous FBI investigation into a Russian oil company. He was planted into the campaign and then because he had so many Russian contacts (compliments of his work for the FBI) he suddenly became a security risk and had to be surveilled...and then everyone he spoke to, emailed or interacted with also conveniently became targets.
Page was a fall guy, the mark who enabled the FISA warrant.