This is Flow, the first of the trio Gabriel caught when he made his way to the human world and became a trainer. Together with Slick and Tumble, they make up "Team Revengeance". They came up with the name and told Gabriel that they were searching for people.
Flow is male and lax natured. He has the highest win percentage out of Gabriel's Pokémon, thanks in part to his high special damage. He has a good boos to it thanks to a custom ability the GM gave him that gives him more damage if he stands still in a fight. I think it is a reference to Final Fantasy 14's black mage.