Quick recap
Shadow Me is told by Kenji between Restore Me and Defy Me: he tries to be a friend to both Juliette and Warner while leading the resistance and watching the group crack. Reveal Me is set between Defy Me and Imagine Me and shows, from the inside, what Juliette goes through away from her friends, held captive and unsure who to trust. The two novellas fill gaps in the new trilogy and set up the final book. Skip it and the series still makes sense, but you miss Kenji's voice, the series' comic relief, telling his own story.

