Despite usually fighting with each other they are very codependent and alike. Both delusional, with Dennis believing he is a "golden god" and Dee believing she is a successful actress, they are both greedy, superficial, and believe they are of a higher class since they grew up rich, since they both went to college and studied psychology they both think they are far more intelligent and superior to their friends. Dee is the older twin, as their mother referred to her as “first is the worst”. Their relationship is toxic and dysfunctional. Dennis must control Dee, ruining almost all of her relationships and Dee dislikes Dennis to the point of hate, yet the two never really leave each other, Dennis lets Dee work at the bar since season one even though she does not own it or even work and Dee lets Dennis and Mac live with her when they burn their apartment down even though they are horrible roommates.
They are the fraternal twins of Barbara Reynolds and Bruce Mathis, raised by Frank Reynolds. Dennis and Dee appeared to get along when they were kids, Dee being Frank's favorite and Dennis obviously was their mother's favorite. They went to Jersey Shore multiply times on vacation and remember loving it, they went to a Nazi training camp together with their grand father, and both hated Frank for when he would buy all the things they wanted for himself on Christmas.
In (The Gang Gets Racist) Dennis and Dee seemed to have a much more normal relationship Dennis teases his sister in a healthy way and Dee even tells Dennis she loves him, though at the end of the episode Dee tries to manipulate him for her own needs by getting Dennis drunk and hiring some of her acting friends into making Dennis believe he slept with them but they called and canceled. In (Charlie wants an abortion) Dee seems to be annoyed of Dennis having no morals and calls him a scumbag. They go to a protest but end up being on different sides, though this seems not to bother them and Dee is actually happy when he leaves to go to the other side of the fence and happy when it backfires on him. In (Underage Drinking : A national concern) Dee begins a relationship with an underage boy and Dennis disproves of it and even seeks to destroy their relationship, knowing her true motives. Though when Dee finds out Dennis is taking an high school girl to prom Dee believes he is a hypocrite. Though neither of them go to the dance because both of their dates dump them. In (Charlie has cancer) the two don't interact very much except when Dee tries to convince Dennis to hire Artemis at the bar so she doesn't have to work. In (Gun Fever) Dee begins dating Colin (a thief who robbed their bar) Dennis becomes suspicious of Colin and figures out he was the one who robbed their bar, Dennis, Dee, and Mac team up to catch him but fail and accidentally shoot charlie, while at the hospital, Colin robs them again. In (The Gang finds a dead guy) The twins barely interacted in this episode though it was revealed that their grand father, "pop, pop" was a Nazi. In (Charlie got molested) the twins believe that charlie was molested by their former gym teacher and set out to prove it, both believing they are equipped to handle the situation since they studied physiology, for the most part they fight with each other though this could be another one of their sibling rivalry's, they continue to try an "help" charlie though Charlie says they ruined his life, when Mac questions them or researches the subject they mock and fight with him.
- When Dee and Dennis get an email from Bruce Mathis claiming to be their real dad, the set up a dinner with Frank and their mom to confirm that Frank is their biological dad. They confront Frank, who shows up early, saying they need to talk to him about something. Frank misunderstands and accuses them of "banging each other" and that it is "perverted". (Dennis and Dee Get a New Dad)
- In Sweet Dee's Dating a Retarded Person, Dennis repeatedly attempts to convince Dee that her current boyfriend is intellectually disabled. When Dee can no longer fight the seeds of doubt Dennis has planted in her mind and breaks up with Lil' Kevin, Dennis tells her that he was only saying it to mess with her because he thought it'd be funny. Once Dee has stormed off however, Dennis reveals to Mac that he lied and does genuinely believe Kevin is "totally retarded".
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- In Mac's Big Break they start a podcast, occasionally arguing but overall having fun. In Mac and Charlie: White Trash, they attempt to prove they are high class by joining the country club together, but when they can't, they go to the public pool. In Dee Gives Birth, Dennis seems to show care and concern for Dee and her baby.
- Dennis and Dee team up for the game of Chardee MacDennis, even unveiling a team poster they have made which shows the two of them in a pose that looks rather uncomfortably like the cover of a bad romance novel. (Chardee MacDennis: The Game of Games)
- When Charlie tells Dee that Dennis is receiving a handjob merely seats away from them in the cinema, Dee's only reaction is to congratulate Dennis. (Thunder Gun Express)
- Dennis reveals that he keeps detailed psychological dossiers on all the other members of the Gang. His dossier on Dee was the first one he began, as a small child; the therapist he shows his dossiers to remarks that the first entries in that dossier are written in crayon. (The Gang Gets Analyzed )
- Dennis doesn't join in Frank, Mac, and Charlie's plan to encourage a broken-down Dee to pursue her stand-up comedy career, because he says it will result in Dee failing miserably and probably being the last straw that pushes her to suicide. Instead, he tries to find her a "perfect select": a man who will "take her off our hands." Yet another way for Dennis to continue to assert control over Dee. When it appears that Dee's stand up career is starting to work out, he becomes extremely distressed. He eventually comes to the conclusion that he is the perfect select. He begs Dee to let him join her in her life of fame, even telling her he loves her, but Dee ultimately rejects him. (When the whole thing turns out to have been an elaborate joke on Dee, which Dennis was not in on, it appears that he himself has a breakdown and The Gang now believes that Dennis may kill himself.) (The Gang Broke Dee)
- Dennis has no qualms about demonstrating to Dee, Mac, and Charlie that he "can go from flaccid to erect at a moment's notice". Dennis's jeans prevent the truth of his claims from being easily ascertained, prompting Dee to frown and stare.
- When the gang is trapped in the room on the cruise ship and are about to drown to death, Dennis tells Dee that he loves her, to which Dee replies: "Whatever." and sinks to her presumed death.(The Gang Goes to Hell: Part Two)