adjective / noun
ancillary
AN-suh-lair-ee/ˈænsəleri/
Providing support to a main activity, system, or organization.
The pianist provides ancillary musical support to the soloist.

Etymology
A handmaid supplied the original image of subordinate service. Applied more broadly, ancillary describes something that assists a main activity rather than being its central function.
In context
have her as almost like the mission Giver character or an important but ancillary character and just invent a new protagonist
More examples
- Ancillary staff handled records and scheduling.
- The manual included several useful ancillaries.
- Parking fees supplied ancillary revenue.
Forms
ancillary · ancillaries