I realize what I'm missing in my stories!

As a writer, I've realized that that haven't added any history behind my characters, or at least not enough. All I've really concentrated on is my characters and well that's it. I just realized that I skipped so so many steps in making a good story. Having good lore is always a requirement in a story. I should look more into this subject. The character I am doing is a starfish so I should look into poseidon. And his sidekick is a servant of Reaper, so I should look into that as well. It will take time, but I am prepared to do so. I am just wondering what I should use for this task. MAYBE READING MANGA WILL HELP!