IPA: /rɪˈwɔːrdər/
KK: /rɪˈwɔrdər/
A person or thing that gives a reward or something of value in return for a good action or achievement.
The teacher was a great rewarder, always recognizing her students' hard work with praise and prizes.
Rewarder → It is formed from "reward" (from Old French *recompenser*, meaning to compensate or repay) and "-er" (meaning a person who). A rewarder is a person who gives a reward or compensation.
Think of someone who 'compensates' ('recompenser') others for their efforts — that's why a rewarder is someone who gives rewards.