What part of the word ruler means someone who?

The part of the word "ruler" that means "someone who" is the suffix "-er".

This suffix is often added to verbs to create nouns that refer to a person who performs that action.

For example:

* rule (verb) + -er = ruler (someone who rules)

* teach (verb) + -er = teacher (someone who teaches)

* paint (verb) + -er = painter (someone who paints)