Complex sentence: In this role, Pierluisi serves all United States citizens residing in Puerto Rico in an island-wide at-large congressional district, being their sole representative upon Congress, specifically within the House of Representatives.
Split into simpler sentences: In this role, Pierluisi serves all United States citizens residing in Puerto Rico in an island-wide at-large congressional district. As the delegate for this district, he is the sole representative for those citizens upon Congress, specifically within the House of Representatives.