IPA: //ˈkaʊntərəˌtækər//
KK: /ˈkaʊntərəˌtækər/
A person who responds to an attack with their own attack.
The counterattacker quickly turned the tide of the battle with a well-planned response.
Counterattacker → It is formed from "counter-" (meaning against) and "attacker" (from the verb "attack", which comes from Old French *ataquer*, meaning to assault). The word "counterattacker" refers to someone who attacks in response to an initial attack.
Think of someone who is acting 'against' ('counter-') an 'assault' ('attack') — that's why a counterattacker is someone who strikes back.
No commonly confused words.