In The Ladies' Paradise by Emile Zola, there are two hugely key characters to know about. The first is Denise Baudu, a young woman who travels to Paris and starts working in a department store. She has two younger brothers that she cares for. At the store, conditions are rough, and she has to deal with sexist standards enforced by the other key character of the book, Octave Mouret.
Octave owns the store and is not the type of guy you'd want to be friends with. He's trying to make his store have a complete monopoly, and does so by driving out smaller businesses and using shady business tactics to get specifically women to pay more in his store. Much of the story centers around the interactions between these two, who eventually marry near the end of the novel. If you only know about two characters in the book, these are the two to know.
There are a lot more characters to be mindful of when it comes to Never Let Me Go. The protagonist is Kathy, who is a clone. The story is through her eyes, and the other characters are important due to their relationship to her. She is basically the ideal protagonist and is stocked with good traits: morality, kindness, etc. Another important character is Tommy, one of Kathy's best friends, who is defined by his emotional changes throughout the book. He is not creative and struggles with this throughout the book. Some other important characters include Ruth, Kathy's best friend and Tommy's girlfriend; Miss Emily, the Headmaster of the Kathy, Tommy, and Ruth's school; and Miss Lucy, another teacher at the school who the kids all like.
Frankenstein's character list is pretty simple. There's Dr. Victor Frankenstein, who begins the story via a series of letters he has written. He's a scientist who created the monster that is the main focus of the book. Then, there's of course the monster he created, which is referred to as a creature, an abomination, etc. (but never "Frankenstein," because that's the doctor himself). Henry Clerval is Victor's childhood friend. De Lacey is a blind man who the monster becomes friends with because he cannot see how hideous he is; later, De Lacey's children see the monster and expose who he is to De Lacey. This teaches the monster a ruthless lesson that he can never find acceptance.
There are definitely more characters in each story, but there's a cursory glance. Hope that helps!