The main themes in 'Everything was a mistake light novel' might include the fragility of relationships. When everything is considered a mistake, it's likely that relationships have been damaged. So, the novel could be about how the characters deal with the broken relationships, whether they try to repair them or move on. It may also touch on the idea of fate. Maybe the characters start to question if all the mistakes were just part of some pre - determined path.
In the 'Everything was a mistake light novel', we can assume there is a central character who is likely to be a complex individual. This character might have a group of friends or family members around them. Some of these secondary characters could be the ones who are affected by the main character's mistakes. For example, a sibling who has to deal with the consequences of the main character's actions, or a friend who tries to support the main character through their journey of realizing and dealing with the mistakes.