Multiplayer ?
A person who looks on the gloomy side of things is pessimistic.
To get to the other side?
In a singles match there is one person on each side of the net, for a doubles match there are two on each side.
flip side?
Optimist/optimistic
An optimist.
An optimist.
Each player starts off on the deuce side (right side of the court) at the beginning of the game and alternates between deuce and ad (left side) every point, but every game always starts on the deuce side.
Tennis played with two players on each side is called doubles tennis.
Weatherstripping
An optimist.
pessimist