Your answer may come from clarification. If you plan to criticize a published author’s work, then you are fine. Literary criticism allows you to discuss (write about) a writer’s characters in published works.

If by ‘write about’ you mean use an author’s characters from a published work in stories of your own invention, you stepped into the world of Fan Fiction and things get murkier. You should research this yourself or under advise of a lawyer. Basically, you can do it, but you, nor anyone else, can profit from it. You (most likely) can not publish such a work without getting permission from the original author.