John Fierro is the second-in-command of the Fifth Column, initially as Eli Cohn's most trusted aide in the Column. Fierro fought beside Cohn and had the highest of respects for him and those he considered highly.
When Cohn, Fierro, Erica Evans and others were trapped in a stand-off with Visitors and FBI, Cohn, realizing he had to give his life for the cause and the survival of the Fifth Column was in jeopardy, asked Evans, who he appointed as his successor to lead the Column, to make sure that Fierro, as well as other key Column members, got out of the building with the rest of the hostages, as he possessed all the contacts and information Evans would need. Fierro was successfully evacuated from the building, disguised as a hostage. ("Siege")
Following Cohn's death, Fierro helped Evans and her team get in touch with the global network of the Fifth Column. Although the international network of lieutenants were (initially) not convinced by Evans's ability to lead, Fierro, abiding by Cohn's last wishes were that Evans lead the Column, stood beside her and spoke up for her defense.("Birth Pangs")
On the official website, Fierro, identifying himself as "Lieutenant John Fierro", maintains The Fifth Column Journal - a collection of entries made after each Season 2 episode, giving a larger and more global perspective of other Fifth Column cells operating around the world.