It could be a character from a different dimension or realm. This character might have some sort of connection to the Miraculouses and sees potential in Hawkmoth. They decide to help him, thinking that they can redirect his actions towards a more positive outcome, or at least a less destructive one.
I'm not sure specifically as I haven't read this particular fanfiction. But the guide could be someone with special powers or knowledge. Maybe Hawkmoth gets help like strategic advice or some form of magical assistance to further his evil plans.
Hawkmoth's character development in the 'hawkmoth gets help from a guied fanfiction' could vary. Initially, he's all about his evil plans. But with the guide's help, he could become more empathetic. For instance, the guide might show him the consequences of his actions on the people of Paris. This could make him hesitate in his villainous deeds. He may also start to adapt new skills or knowledge from the guide, which could change his modus operandi. This new influence could make him a more three - dimensional character, with a mix of his original villainous traits and some new elements introduced by the guide.
Well, in some fanfictions, Hawkmoth might use false promises. He could promise Cat Noir something like the power to save a loved one in exchange for his help.
Maybe the guide is a time - traveler. Someone from the future who has seen the consequences of Gabriel's actions and comes back to try and change the course of events. This time - traveler would know all the ins and outs of what's going to happen and could guide Gabriel towards making better choices. They could show him the destruction his actions as Hawk Moth will cause and offer him an alternative path to achieve his goals in a more positive way.
One possible plot could be that Gabriel Agreste, in his search for a way to achieve his goals (maybe related to his fashion empire or his secret identity as Hawk Moth), gets help from an unexpected guide. This guide might be a character who has unique knowledge of the Miraculous or the hidden aspects of Paris. For example, the guide could be an old acquaintance who has connections to the magic that surrounds the Miraculous. They might lead Gabriel to discover new powers or secrets that could change his approach to his plans.