Penny Fleck's Boyfriend

Penny Fleck's Boyfriend is the unseen overarching antagonist of the 2019 psychological thriller film Joker, the first installment of the DC Black film series.

He is the former boyfriend of Penny Fleck, a mentally ill woman who claims to have been on an affair with Thomas Wayne. While still on a relationship, he severely abused Penny's son Arthur and possibly Penny as well, leaving traumas onto Arthur that eventually come to take its toll by the time of the film's events.

Biography
When Penny Fleck was younger, this man became her boyfriend and went to live together with her in Gotham City. Supposedly, Penny had an affair with Thomas Wayne and had a son of his, who was named Arthur Fleck. If this was true, however, it's unknown if Penny was already in a relationship with this man or she hadn't met him yet. According to the Arkham State Hospital's files, Arthur was adopted by Penny but her boyfriend severely abused Arthur, leaving him with traumas and physiological damage until he developed a neurological condition that caused him to develop uncontrollable laughter in unsuitable moments. He likely abused Penny as well both physically and mentally, just like Arthur.

As a result of the abuse Arthur suffered, Penny was committed to the Arkham State Hospital for her mental instability and her inability to protect Arthur from her boyfriend despite Penny's claims that Arthur was Thomas Wayne's son and that they had been in love. It's completely unknown what happened to Penny's boyfriend, but it can be deduced that he was arrested by the authorities and sent to prison for the abuse he inflicted on Arthur unless he wasn't found.

Eventually, Penny somehow managed to regain Arthur's custody and Arthur suppressed these memories, living with Penny together in an apartment. However, in 1981, after Arthur realizes his true parentage and goes to the Arkham State Hospital to discover if Thomas Wayne is saying the truth that Penny is mentally unstable, Arthur finally remembers the abuse he suffered at the hands of Penny's boyfriend and that Penny lied to him the whole time. As a result, Arthur finally loses what little remains of his sanity and kills Penny before becoming the Joker and triggering a mutiny against the rich and the elite on Gotham which leads to Thomas Wayne's death. Thus, there is one certain thing about Penny's boyfriend: he is indirectly responsible for making Arthur the person he is now.

Trivia

 * While not confirmed, the revelation about Penny Fleck's Boyfriend may be a reference to some of the stories about his early life the Joker uses to tell in the comic books, where he assures that his father was abusive towards his mother and him.
 * He may have been abusive due to Penny's claims of having an affair with Thomas Wayne.
 * Despite being unseen, he is essentially the main reason for Arthur's misery and the events that happen throughout the film.