

The dad/father of Xavier Riddle and Yadina Riddle is shown in a brief flashback at the end of I Am Tomioka Tessai, holding a teddy bear when he was younger.
This happens after Yadina Riddle asks her mom about the history of the family quilt.
As an adult, he appears as just legs at the end of I Am Cleopatra while Xavier tasks to him and mom about seeing a supermoon, where they are later seen seated.
Yadina's great grandma likely comes from his side of the family, since she has a dark skintone while Yadina's mom has a lighter skintone.
A sketch of him is shown in I Am Confucius when Xavier relates about soup.
The Riddles' parents are also shown from behind during a flashback to a previous family picnic at the start of I Am Rukmini Devi. By the end of the episode they are shown in side-profile walking to the next one.
After this, when a series of photos next to the tree are shown over several generations, it ends with a photograph of a toddler Xavier holding a baby Yadina as their dad is visible in the background. Preceding that last photo is a photograph of their dad when he was around Xavier's age with two siblings (the Riddles' dad's sisters), as well as the three siblings' parents, including Riddles' dad's mom (their paternal grandmother)
The Riddles' dad's dad (Xavier and Yadina's paternal grandfather) was shown in the preceding photo as being the son (and presumably only child) of their two great grandparents: the Riddles' dad's dad's mom and Riddles' dad's dad's dad.