Partisan noun – One who follows the opinions or teachings of another. Extremist and partisan are semantically related. Sometimes you can use "Extremist" instead the word "Partisan" as a noun or an adjective. Nearby Word: partisanship
It is interesting that on the old Israel Insider website - a very partisan propaganda site for Israeli expansionism - many comment pieces were written by Northern Irish Unionists/Loyalists, including a regular commentator known for his association with extremist Loyalist factions.