Incidental

1. Loosely associated; existing as a byproduct, tangent, or accident; being a likely consequence.
2. That character, though colorful, is incidental to the overall plot.
3. Occurring by chance
4. (physics, of radiation) Entering or approaching, prior to reflection (more frequently incident).

(existing as an accident): accidental, contingent; circumstantial

(occurring by chance): accidental, serendipitous; accidental

(existing as an accident): inevitable, necessary, impossible

(occurring by chance): inevitable, intentional