Penny Fleck's Boyfriend

Penny Fleck's Boyfriend is an unseen yet significant antagonist in the 2019 psychological thriller film Joker, the first installment of the newly launched 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 on 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 really true, however, it's unknown if Penny was already on a relationship with this man or she hadn't meet him yet. According to the Arkham State Hospital's files, Arthur was actually adopted by Penny but her boyfriend severely abused Arthur, leaving him with traumas and physiological damage until he developed a neurological condition which caused him to develop an 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 really 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 send 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's one certain thing about Penny's boyfriend: he is indirectly responsible for making Arthur the person he is now.

Trivia

 * Given Arthur Fleck's nature as an unreliable narrator, coupled with the implication that Thomas Wayne was possibly his real father and fabricated the adoption papers, it's possible that Penny Fleck never had a boyfriend to begin with and thus this individual never actually existed or he was some person Thomas hired to back up his claims and commit Penny to the Arkham State Hospital in order to hide the affair.
 * Despite not appearing in the film, Penny Fleck's Boyfriend essentially serves as The Heavy: it was because of his abusive actions (if we consider that Arkham's files were right) that Arthur Fleck was left with several traumas which eventually trigger his transformation into the Joker after all the abuse he continues to suffer.
 * While not confirmed, it's possible that the revelation about Penny Fleck's Boyfriend is 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.