Novella writers chose the novella form because it allows them a certain focus ― as opposed to the potential breadth of a novel. For that reason there are often fewer characters in a novella.
There may even be just one character.
The most important tip is that every character must perform a function in the story.
Identify the essential contribution each character will make to the story ― start simple ― does any secondary character help or hinder the main character? Write it down or keep a document with this information in it.
Be sparing with your character’s dialogue and the description of what they look like. Everything they say has to further the plot and create a vivid sense of your novella’s world. That is everything they say, no small talk please.
Try to write character descriptions as a preliminary exercise before writing their scenes. Or write out the descriptions of characters in other novellas.
Work out each character’s role as you would an actor’s in a play.
Like the cast of a play, your actors have limited lines to get across their motivations and their purpose to the story.
When the curtain goes up, every line your characters speak is hostage to the roaring success of your novella!