Great-Great Grandma Riddle (voiced by Teresa Pavlinek) appears in Xavier Riddle and the Secret Movie: I Am Madam President.
She is the one of the great-great grandmothers of Xavier and Yadina.
She co-founded the Museum atop The Secret Museum, and can appear as a hologram projected by Berby despite having passed away.
She shares many similarities to her great-great granddaughter, Yadina Riddle; both wear pink, both make jokes, both like cheese and grape jelly sandwiches, and both like turtles.
It is never clarified whether she is their ancestor on the maternal or paternal side of the family. However since she bore the surname "Riddle" this indicates it would be whichever side of the family (mother or father) the surname originated. That is traditionally the father's side (with wives taking husband's surname) but this is not always practised in modern marriages, and it is never clarified on the show.
Great-Great Grandpa Riddle is the logical name for the man shown beside her in the painting, since they are referred to as "our great-great grandparents". Xavier says they built the ENTIRE museum, which is probably the sole other party who the Tour Guide implies when describing GGGM Riddle as "one of the founders". If the two built the entire thing by themselves, there would be no additional parties. Xavier of course could be simplifying things as they might've just been in charge of construction and had assistance from builders.