In the Volume 3 cartoon series and Freaky Fusion, Frankenstein's monster or Monster of Victor Frankenstein is a simulacrum monster created by the well-known brilliant human scientist, Victor Frankenstein and father to Frankie Stein. Not much is known about him in the Lisi Harrison Monster High book series, because in the Lisi Harrison book series, Viktor Stein, the monster's son, is Frankie's father; but in the doll diaries, Viktor Stein is Frankenstein's monster, and has a habit of screaming: "It's alive!" every time his daughter comes down to breakfast, which Frankie also mentions in her profile bio, by her regular bio.
The Monster of Frankenstein is husband to Viveka Stein in the diaries of the dolls, who is the Bride of Frankenstein in the diaries; but in other cases of Monster High franchise, she is just the Bride of Frankenstein. In the diary franchise, the Bride is Viveka. Overall, Frankenstein's monster even says his father's (creator's) name as "Grandpa" into "Grandpa baaaad!" as a typical stereotypical version of the Monster according to Frankie in "Freaky Fusion". He is the creator of Watzit, Frankie's pet dog-looking simulacrum made of many different animal parts and can do many different things that are particular to the animals he is made of.
In the doll diaries, specifically Frankie Stein's 'Basic' diary, Watzit had been a surprise to Frankie that Viktor (Frankenstein's monster) had revealed as a special project. According to Frankie's Basic diary, Frankie once threw a ball for Watzit to fetch, causing her stitches to come undone, and when Watzit returned both the ball and her arm, Viktor was the one who needed to put her stitches back together.
When Frankie found out that her father was working on a special project, he and his wife, the Bride, started to laugh when Frankie's mother told her husband about it later, causing the young Frankie to wonder if other teenage monsters had parents who do other things. Every time The Monster screams "It's alive!" about Frankie's arrival downstairs and stops laughing, he slaps his knee and says: "That never gets old," causing Frankie to believe that breakfast is ovverrated.
Frankenstein's monster overall is shown to be a kindhearted, good-souled, compatiable, fun-loving monster and loving father who enjoys to see his daughter happy, but can sometimes get a strict side like all other parents when the time is right.