Whenever an infant or incompetent person has a representative, such as a general guardian, conservator or other like fiduciary, the representative may sue or defend on behalf of the infant or incompetent person. If an infant or incompetent person does not have a duly appointed representative, or if justice requires, the infant or incompetent person may sue by a next friend. The court shall appoint a guardian ad litem to defend an action for an infant or incompetent person who does not have a duly appointed representative, or whenever justice requires. The court may, in its discretion, allow the guardian ad litem a reasonable fee for the guardian's services, to be taxed as costs.