IPA: /əˈsɪstə/
KK: /əˈsɪstər/
A person who helps or assists someone else in a task or activity.
The assister provided valuable support during the project.
Assister is formed from "assister" (from Old French, meaning to help or support) and the suffix "-er" (meaning a person who). The word "assister" means a person who helps or supports others.
Think of someone who is there to 'help' ('assister') — the '-er' just means a person. An assister is someone who helps.