Robotnik is the dean of Fils-Aime University, where Sonic, Tails, and Mario are employed. Once he catches wind of Sonic's antics in the lab, Eggman (as Sonic mockingly calls him) throws the three of them out of the University...but not before absolutely chewing out the trio. 

Eggman's role is minor, as he only has a few spoken lines as he absolutely rips into the team of three (but his anger is mostly directed at Sonic.) 

As for voice direction, I'd like him to sound similar to the mainline games (Mike Polluck) but I wouldn't be opposed to other takes of the character from throughout the years!

Voice clips for reference: https://youtu.be/pCDM4PwkMuE