I don't write novels, but lately found apostrophe (gnome) and ghostwriter (KDE) which are intended to write using markdown, and have a UI intended to allow you to focus on writing. You can later use git to manage versions and backups (in a remote repository).

If you want something more focused on relationships, and regarding the answer from another user suggesting Obsidian, you might use also logseq, but I didn't use it yet (but hear a lot of positive vibes around it).