Isaac Dian (アイザック·ディアン Aizakku Dian) and Miria Harvent (ミリア·ハーヴェント Miria Hāvento) are the bumbling thief couple of Baccano!.
Isaac and Miria are a pair of clueless and eccentric partner-in-crime lovers who dress up in costumes and commit strange robberies - such as stealing candy and even the entrance door of a museum (so people "couldn't enter there anymore") - in Prohibition-era United States. In 1930, the duo attempt to steal the entire household of the mafia families of Genoard (which they succeed) and Martillo, incident on which they become unintentionally immortal after mistaking the Grand Panacea for alcohol and drinking it at a celebration. The next year, after attempting gold mining in California without any luck, they board the Flying Pussyfoot express train and survived the bloody hijacking that occurs. Isaac is arrested for various thefts in 1934 and imprisoned on Alcatraz, but released in the same year, later reuniting with Miria. The pair continue to live happily, finally realizing they are immortal 67 years later, in 2001, after stealing some cellphones in Manhattan.
They both individually possess true immortality, meaning they don't age or get sick, they don't need to eat, drink, or breath, and whatever injury they get will regenerate/heal within a matter of seconds, even incineration or decapitation. This immortality extends to their mind and spirit as well as their body. Immortality aside, they possess an endlessly large amount of good luck, having escaped numerous dangerous situations throughout the series without serious injury (it took them roughly 70 years to realize they were immortal), things go their way. They're both very charismatic, eloquent, and "sprinkle happiness" upon anyone and everyone they meet without noticing. They have easily made friends with all sorts of people, Isaac even became friends with Ladd Russo while in prison. They also possess indomitable will as is suggested by their headstrong personalities, or the fact that they haven't changed regardless of how much time has passed. Isaac has also demostrated a couple of times that he possess incredible observational and deductive skills, as well as enhanced intelligence. They are both cute attractive, possessing flawless smooth fair skin, and being "well-endowed". They durable, possessing enhanced strength,speed, reflexes, senses, dexterity, agility, and are proficient in the use of firearms, blunt instruments and unarmed combat.
- They resemble the real-life thief duo Bonnie and Clyde to a more ridiculous extent, as Bonnie and Clyde's attempts at organized crime often went hilariously awry.
- Numerous publications in various media have been written on the subject of Isaac and Miria's characters. Most described them as the most entertaining characters and commented on how the series would not be the same without them. One reviewer felt that they are the protagonists of the series, which features an ensemble cast. They were awarded "Duo of the Year" in 2009 by Anime News Network.
- Isaac and Miria appear in the series' two drama CDs, where they survive the Flying Pussyfoot incident in 1931 Local Chapter ・ Express Chapter The Grand Punk Railroad and hope to meet true revolutionaries in a small Mexican village when Firo Prochainezo witnesses the 53rd death of Pietro Gonzalez.
- They are playable characters in the video game and characters in the two-volume manga.
- They are used in the anime series to comment during the preview of the next episode; though their observations aren't usually very perceptive.
- The pair also made a brief appearance in an episode of the Durarara!! anime series, during which they are in modern-day Ikebukuro, Tokyo and attend the first meeting of the Dollars, an anonymous online gang. This seems to settle a debate over whether the worlds of Baccano! and Durarara!! are connected. They also appeared in the following OVA episode seperate from the main series, where they comment on the supposed UFOs seen shooting across the sky. These are actually yakuza members covered in paint that were thrown out a three story window by Heiwajima Shizuo.