The term factionist describes a person who supports or belongs to a political or social faction. Synonyms for this word could include:
1. Partisan
2. Sectarian
3. Groupie
4. Adherent
5. Devotee
6. Zealot
7. Radical
8. Activist
9. Strident
10. Extremist
Each of these synonyms highlights different aspects of a factionist, from their deep commitment to a cause to their divisiveness and tendency to be radical or extreme in their beliefs. While there may be subtle differences between these synonyms, all suggest a strong allegiance to a particular group or ideology.