Non-partisan or independent.
this person is called a soldier
A solider
A turncoat.
Admiral
A person opposed to slavery was called an abolitionist. An abolitionist can refer to someone ho supports an end to anything, but was used specifically to refer to those opposing slavery.
Independent
Uncooperative?
The person could be termed as being non-political. Or politically impartial. Or even sensible!
Democratic Party and Republican Party( you can also call it Grand Old Party )
In the United States we call them the Democratic party.
Bland.
party animal
You could call them a party animal. The life and soul of the party opposed to a party pooper.
Party Pooper
You could call them a party animal. The life and soul of the party opposed to a party pooper.
The Host
Guest of honor