How to find out who redeemed my Apple Gift Card?

If you purchased the gift card from a store that keeps the cards out in the open rather than in a secure location, then it was probably redeemed by a thief who got the code, re-covered it, and then waited until someone bought the card and immediately redeemed it. This is a common crime in stores where the cards aren't kept secure. Apple cannot tell you who redeemed the card, and your only recourse is to go back to the store you bought the card from and complaining to them.

In the future, do not buy a gift card of any significance - Apple gift cards, Amazon cards, prepaid credit cards, etc. - from any store that just leaves the cards out on a publicly-accessible rack.

Regards.

.... I want to know the person.

It's quite likely that you already know them.

Anyway, Apple will not reveal information about transactions on other people's accounts. You can follow the steps in here:

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support