Is it the team rocket boss?
As Ash's story ark has come to an end, it is unlikely we will know who he is, although it was considered to introduce him depending on how the story unfolds. Ash has never met his father, as he left home after Ash was born and hasn't returned. It's implied Ash has found himself in a family with Professor Kukui and Professor Burnet.
Relatives: Delia (mother), Unnamed father (EP002), Unnamed grandfather (EP002, Japanese version only).
His father has not been introduced. The anime suggests that his father was once a Pokémon Trainer who began his journey in Pallet Town and that he is still living. In the original, he is also confirmed to have a grandfather, whose son followed his footsteps. An interview with the anime's storyboard artist Masamitsu Hidaka confirmed Ash's father is a Trainer on his own journey. According to Hidaka, Ash's father may or may not be introduced in the anime, depending on its necessity to further Ash's maturity and development as a Trainer. On the other hand, according to Takeshi Shudō's novelization, Ash's father left home to go on a journey right after Ash was born, and never returned since. As he never registered his name in the Pokémon Trainer's list, his whereabouts are unknown.
However, during his time in Alola, Ash's family expanded with Professor Kukui and Professor Burnet, as shown in The Shape of Love to Come!, when Mina was making a family portrait of them and agreed with Lana when she called the Professors his Alolan family. Ash also gets an Alolan brother in Professor Kukui and Professor Burnet's son, Lei, as seen in That New Old Gang of Mine!.
Source
We don't know for sure. A google search gave me sites listing theories for Sir Aaron, Giovanni (the team rocket boss), Bruno, or it could be an unknown character. As far as I know, Gamefreak has never confirmed.
