Basically, a fiduciary is someone on whom the law imposes the highest duty to act on behalf of another like an executor, administrator or trustee.
Fiduciary relationships can be formal such as partners, agents, executors, administrators, trustees, a holder of a power of attorney, etc.
Fiduciary relationships can also be imposed by law because of the relationship of the parties such as domanance by one and weakness by another, where one has gained the trust of another to handle their affairs, etc. In a 2018 case, the Court of Appeals held that a funeral home had a special relationship (fiduciary relationship) with the family concerning the handling of a body. In the case, the funeral home picked up, transported and displayed to the family the wrong body. 08-17-00151-CV.