A young apprentice who wants to learn from the poison dragon slayer. This character can bring out the nurturing side of the slayer as they teach the apprentice about the dos and don'ts of slaying dragons and handling poisons.
A rival slayer who uses different methods and doesn't believe in the poison - based approach. This creates tension as they constantly compete with the poison dragon slayer, whether it's for the glory of slaying a dragon or for the limited resources related to dragon - slaying, like rare antidotes and weapons.
There could be a wise old mentor who was also a poison dragon slayer in the past. He can guide the main slayer with his years of experience and knowledge about different poisons and dragons.