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Yes, you can develop secondary characters, and should to the extent your narrative has room. While they are multi-volume arcs, David & Leigh Eddings's Belgariad and Malloreon series are good ...
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Source: https://writers.stackexchange.com/a/28636 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/28636 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Yes, you can develop secondary characters, and should to the extent your narrative has room. While they are multi-volume arcs, David & Leigh Eddings's Belgariad and Malloreon series are good examples of this. The main character is Garion, later Belgarion. Other major protagonists are his aunt Polgara, grandfather Belgarath, and eventual wife Ce'Nedra. There are five or six secondary protagonists who travel with them on their quest, and _all_ of them get enough backstory and character development that you know who each person is individually and separate from Belgarion's story. You don't want to overwhelm a single book with eight characters' backstories, but each person should get enough presentation that they are distinguishable. And please try to give everyone a personality and at least a suggestion of a life _which has nothing to do with the hero._ This is critical for a love interest. The Love Interest should have a life which starts before s/he meets your hero. Family, friends, hobbies, history, a job. If everyone in your book exists solely to perform a function for the protagonist, your book may feel flat. You don't have to have 20 heroes, but yes, some fleshing out is good.