Robert Gru

Robert Gru was Felonius Gru and Dru Gru's father, Marlena Gru's ex-husband as well; he was also the adopted paternal grandfather of Margo, Edith and Agnes, the father-in law of Lucy Wilde, and the son of John Gru.

During the event of the Despicable Me 3, he is revealed to be a legendary supervillain known as "The Bald Terror" with more success than both Gru and Dru.

Despicable Me
Robert has only been seen in a picture on the Gru Family Tree but he never appeared in person during the first film and wasn't even at his son's wedding near the end of the second film, or with him and his wife at the Villain-Con in the film Minions.

Despicable Me 3
After learning the existence of his twin brother Dru from Fritz, Gru questions his mother who informs him that Dru does exist, and after she divorced with Robert they never met again. She had told Gru earlier that Robert died, despite the fact that he was still alive.

Robert later moved to Freedonia with his son and began his career as a legendary supervillain, using a hog-raising business as an operating front. Robert remains satisfied with Felonius Gru's acts of villany, though he is disappointed Dru Gru due to his lack of talent.

Sometime before the events of the film, Robert died of unknown causes. His son Dru Gru inherited his mansion Dru's butler, Fritz, found Gru's House to tell Gru the existence of his unheard relatives.

Appearance
The drawing in the family tree in Gru's House depicts Robert as an overweight and an upper-class with black hair and mustache, also has a strange-looking face and like his son, Gru and his ex-wife, Marlena a long, pointed nose.

In another portrait of Robert in Dru's mansion, his true appearance reveals that he had an uncanny resemblance between his sons with the exception of a bald spot and shoulder length blond hair.

Trivia

 * Robert's appearance shows that Gru and Dru inherited most of their physical traits from their father, though Gru seems to have inherited his hair color from his mother.
 * In the family tree seen in Gru's house, Robert looks different from Despicable Me than he does from the portrait in Despicable Me 3. In the family tree, he is depicted with a short curly hair, black hair, and short sideburns, with a horseshoe mustache. His style of dressing also seems different. In his portrait shown in his villain lair underneath his mansion in Freedonia, he has a bald spot, short, straight blond hair, and short sideburns.