As mayor, Kline is “slick” and “sleazy,” putting a higher priority on what makes him look good than what’s good for the people of Hawkins.
The news that Cary Elwes had joined the cast as one of the three new characters announced for the third season was confirmed by Deadline, less than a week ago.
In the picture Elwes posted, he is posing with Joe Keery, who plays Steve.
He not described in favorable terms, so you can probably cross him off the list of potential good guys.