IPA: /əˈsɪstər/
KK: /əˈsɪstər/
A person who helps or supports someone in a task or activity.
The teacher had an assistor to help with the students during the exam.
Assistor is formed from "assister" (from Latin "assistere", meaning to stand by or assist) and the suffix "-or" (meaning one who). The word "assistor" refers to one who stands by to help or assist.
Think of someone who 'stands by' ('assister') to help others — that's what an assistor does.