I think it is fair to say that during our lifetime most of us have had the misfortune of having a toxic friend. Whether it was from school, university or work. Whenever there are a group of friends, especially a group of females, there is always peer pressure to conform. You knew what position you held in the group, whether it be the comedian, the geek or the leader, and there was no diversion from that. Every member of the group had their label, and there was an unspoken rule to act accordingly or be judged by the others in the group, especially the leader. If your novel revolves around such a group of friends, a great way to create conflict is to introduce one member of the group with a poisonous or toxic personality. Who do toxic friends normally associate with and what are the warning signs you can include when building a backstory for this character? 

Poisonous personalities have certain behaviour patterns, some are obvious and others subtle. They will befriend you and show kindness to your face but act in a totally opposite manner when you are not around. The subject of conversation has to revolve around them always. They love being the centre of attention and if the topic of conversation veers off to a subject they know little about, they will immediately try their best to divert it back to themselves. They are terrible listeners and will often finish people's sentences. A poisonous friend only listens to a conversation so they can interject with their own experiences. They always believe they know the best course of action, even in circumstances where they have no expertise. Areas of conflict could arise if another member of the group begins to think for themselves, and the poisonous character will do their utmost to turn the other members of the group against them.

An important point to remember is that you may often find a toxic character will hate being criticised; this is often attributed to low self-esteem, so consider the personal history of this character. Although they seem confident on the outside, what is their family background? What event in their past caused the shift in personality? Toxic characters believe everything is a competition, so whatever the other characters' experiences, they will have to outshine them with a bigger and better story (even if they have to lie). If another member of the group receives some good news, the green-eyed monster will raise its ugly head and the toxic character will try everything to belittle or undermine the other person's achievements.  

How could you resolve the relationship between the toxic character and the character that is being undermined? Remember the characters have to change somehow by the end of the story. Would the toxic person be ousted by the group and abandoned? Would the undermined character discover the abusive background of the toxic character and help them change their ways or would the toxicity end when they risk losing something important in their lives? For a toxic person to thrive, the supporting characters have to accept the behaviour, so it is important for you to consider this when creating their backstories. Are they insecure characters and have been subject to manipulation since childhood? Does the toxic character have any information they are using to maintain control? Allow your imagination to run wild; make sure there are areas of conflict, where the toxic character somehow manages to escape detection until the conclusion, when they are forced to make necessary changes or because their belief system has changed.