maintainer(n = noun.person) sustainer, upholder - someone who upholds or maintains; "firm upholders of tradition"; "they are sustainers of the idea of democracy"
is a kind of admirer, booster, champion, friend, protagonist, supporter
Derived form verb maintain₁

   One who maintains. [1913 Webster]

maintainer, n.
1 a person or thing that maintains.
2 (also maintainor) Law hist. a person guilty of maintenance (see MAINTENANCE 3).