Defenders are alters who defend a system and its members, whether it be from the outside world or from in-system issues. Defenders act as guardians for others in the system or the body. There are defenders for different purposes or reasons, often steming from the trauma that caused the system or the trauma the system experienced. There can be outer defenders such as physical (From violence) and sexual (From assault), and in system defenders. Those are just the ones I know exist though. There are likely more than just those kinds.