A litigation representative must not take a step in the proceeding unless the following documents have been filed:
(a) the litigation representative's consent;
(b) a certificate, including a statement that the litigation representative has no interest in the proceeding that is adverse to the interest of the person under a legal incapacity, signed by:
(i) if the litigation representative is a lawyer--the litigation representative; and
(ii) if the litigation representative is not a lawyer--the litigation representative's lawyer.