Johnny Tag is the clone of Johnny Gat and was created by Saints fan Jimmy Torbitson. He cloned him by "collecting shit" that Gat had touched but it went horribly wrong. He did not have memories of the events after Aisha's death, and believes that she was only recently killed by The Ronin.
The Trouble With Clones
The Protagonist, Pierce and Jimmy attempt to calm Tag down. When Jimmy calls The Protagonist over to his house one last time, believing he has a solution, Tag arrives, knocks The Protagonist out, and kidnaps Pierce. The Protagonist finds Tag along with Pierce on the Magarac bridge being attacked by the SNG. After a battle with the National Guard, The Protagonist makes their way up to the bridge to reason with Tag and successfully convinces him that The Protagonist and Pierce are his friends.
The existence of Johnny Tag was first discovered through leaked achievement/trophies for the The Trouble With Clones DLC, which includes "The Johnnyguard", with the description mentioning the name "Johnny Tag".
Tag's memories stop right before he got revenge for Aisha, although Jimmy used fairly new DNA obtained through saliva from a Saints Flow can that Gat drank. In the first mission of the "Trouble with Clones"[mission?], as he asks "Where Shogo?" This would suggest that he does not remember the events of Rest in Peace. His memory later on becomes more clear, as at the ending scene of "Send in the Clones", he would say, "Saints failed me", referring to his death on Philipe's personal plane in the mission, "Free Falling".
Even though Tag's name is Tag, at the end of the DLC he says "Gat, friend?". This is because up to that point, no one has told him that he is a clone of Gat. So he probably still thinks he is Gat himself.
Tag was cloned with DNA from Gat that resulted in him mostly resembling Gat from Saints Row and Saints Row 2 yet he has Gat's jacket from Saint's Row the Third.
Tag shouting "Stripper pole" is a reference to the conversation between Johnny and The Protagonist after they cleared the underground hotel in Saints Row 2, in which Johnny says it needs a stripper pole.
Tag claims at the end of the final mission that the Saints had failed him. Seeing as his memory goes as far back as to Aisha's death, it is possible the real Johnny Gat blamed the Saints and himself for her demise.
Despite the fact that he survives the events of the DLC and the ending suggests that the story may continue onwards, Tag and the rest of the characters and events of this DLC are not included or mentioned in Saints Row IV.
Johnny Tag's personality (with a little bit of Johnny Gat's personality) and brute size resembles The Hulk.
Johnny Gat's clone, Johnny Tag
Tag on an Achievement/Trophy icon called "The Johnnyguard"
Promo image of Johnny Tag in The Trouble With Clones