Viktor Stein is a 2010-introduced and prose-only character. He is a simulacrum, specifically the son of the Monster created by Victor Frankenstein, and both happily married to Viveka Stein and father of Frankie Stein.
Viktor was the first creature made by Victor Frankenstein, after which he helped create the second, Viveka. His creation likely occured in either Germany or Switzerland and certainly between 1812 and the end of the 19th century, and probably well in the middle of those dates. Viveka was created shortly after him specifically for Viktor to have a companion and, fortunately although somewhat foreseen due to Frankenstein's knowledge of the future, it worked out for them. The Steins did long for a child of their own, but they wished to have the proper circumstances to raise one first. In the meantime, they were happy to occasionally take care of Robecca Steam, the robot-daughter of Hexiciah Steam, a family friend. Though they were eager to learn as much as possible from her to prepare themselves for what a future child of theirs needed, they never overstepped boundaries and kept her well-being as a priority.
Viktor is a very intelligent mad scientist, who dedicates himself to his work when at work. During off-duty hours, Viktor is pretty much a typical dad and family man, both doting and protective when it comes to his family, and often making jokes, such as yelling "It's alive!" when Frankie comes down for breakfast. He tends to be somewhat frugal though, such as using cheap thread to stitch himself and Frankie together.
In the book series, Viktor is a professor at a local university, and is also researching a way to replace lost limbs for amputees. Like the other monsters families who live in Radcliffe Way, he needs to disguise himself as a human using make-up to cover his skin.
Viktor is a large male composite corpse with green skin, stitched together limbs and bolts in his neck which allow him to charge himself with electricity.
In the book series, Frankie considers him similar to Arnold Schwartzenegger. He wears suits and tracksuits which help hide his stitches.
There is a generational distinction between Viktor as he appears in the Monster High book series and the rest of the franchise. In the Monster High books, which are the only source of a name for Frankie's father, Viktor is not the Monster but a creation of the Monster and the Bride, just like his wife. Essentially, he is second-generation creation in the books rather than a first-generation as he is in the rest of Monster High fiction.
Viktor is the first child of Victor Frankenstein, who was presumably aided by Egor, whom the Steins regard as family. Viktor later assisted in the creation of Viveka. Viktor and Viveka made Frankie together after waiting over a century for the right circumstances to raise a child. Frankie has since made two creations of her own: Hoodude Voodoo and gingerbread boy. In Hoodude's case, the Steins acknowledge him as part of the family, but not as part of the family unit and have arranged for him to live with Kindergrubber.
Viktor is close acquaintances with many other monsters in New Salem, including Dracula, Ramses de Nile, Medusa Gorgon, and the Wolfs. He has had a tradition for years to see a group of his friends every Saturday morning at a local die-ner for breakfast.
Viktor used to have a ferocious cat in his younger days. He lost the animal at some point, but it was found well over a century later by Frankie and a few of her friends.
The Steins have a traditional marriage that works fine for them. Viveka considers Viktor to have limited sense for financial priorities and to be somewhat bothersomely old-fashioned, but she admires his intellect and enjoys his warm theatrics.