Spoilers ahead for A Series Of Unfortunate Events Season 3 and the series of books! In Book 11 of A Series of Unfortunate Events, Lemony Snicket defines the word "denouement" as the main climax in a story, tying up loose ends just before the very end. It's no accident, of course, that "Denouement" is also the last name which Daniel Handler assigned to the triplet brothers that own the Hotel Denouement in The Penultimate Peril, the second-to-last installment of the tale of the Baudelaire orphans. In the books, the Denouement brothers — Frank, Ernest, and Dewey (played by Max Greenfield in the Netflix series) — all play important roles, but as is so common with most of Lemony Snicket's important characters, they are shrouded in mystery.
Like the Baudelaires, the Quagmires, the Snickets, and Count Olaf, the Denouement brothers are orphans. Dewey explains to the Baudelaires that V.F.D. took him and his brothers from their home during the schism, when they were just five years old to begin their training. After that, the Denouement parents perished in a terrible fire.
Frank & Ernest Denouement
Frank and Ernest, while central to the operations of the Hotel Denouement and appearing frequently to give the Baudelaires various tasks or instructions, are static characters in The Penultimate Peril. All that the reader learns about them is that one is a villain, and one is an ally. Unfortunately, the Baudelaires can never tell them apart, which adds to the confusion significantly. In the series, this inability to tell them apart leads to even more confusion ahead of Olaf's trial at the hotel.
In the end of the book, when the fire begins to consume the hotel, Violet shouts for someone to call the fire department. Either Frank or Ernest responds with, "Which one?" Sometimes, the loyalties of two different fire departments are as easy to identify as the loyalties of two identical triplet brothers. It's believed that the brothers may have also perished in the fire that destroyed their hotel, but Snicket doesn't confirm it for sure.
What Frank and Ernest lack in substance is completely made up for in Dewey Denouement's character. Dewey becomes a close friend of the Baudelaires, giving them jobs at the hotel and looking out for them as much as he can. Dewey co-founded the hotel with his brothers, but unlike them, he works behind-the-scenes winding the clock in the hotel lobby. In addition, he's also a "sub-sub-librarian" with a penchant for the Dewey Decimal System. Everything in the hotel is organized to this traditional method of library organization.
In the pond outside the hotel, however, lies Dewey's real magnum opus: his underwater library filled with secret information which he hopes to use to defeat all the villains in the world.
Tragically, the Baudelaires accidentally kill Dewey when they drop Olaf's harpoon gun, sending one of the deadly spears into Dewey's stomach. He forgives them for the accident, then cries out the name "Kit" before slipping into his pond.
Because of this heart-wrenching last line, many fans theorize that Dewey is the father of Kit Snicket's child. The Snicket Sleuth on Tumblr points out that the books purposely leave little Beatrice's biology a mystery — Kit clearly had a relationship with both Dewey and Olaf — but the important part is that Kit wanted Dewey to help her raise her baby, not Olaf. In the series, things go much differently, as it is established that Kit's relationship with Olaf was way before she ever met Dewey, and they are planning on running away with each other after the trial.
The Denouement Brothers, and Dewey in particular, are a difficult plot point to accept. It's built right into their name that the end is coming, and the Baudelaires are running out of time to solve all the mysteries they hope to solve. Dewey and his brothers' entrance into the Baudelaires' lives only serves to open up even more questions for the orphans — and the reader — than they had before. And, unfortunately, many of those questions will never be answered.