an enemy / revenge, vengeance / opponent (of the game or conflict) / (archaic, obsolete) marriage partner

的: (historical) (by extension from the red-light district usage) term of third-person reference: that guy, that one

enemy / foe

to rival

to use one's head

use one thing for many purposes / work someone hard

that which is not yet verified, something unidentified

reaffirmation, reconfirmation

reaffirm, reconfirm

(colloquial) current boyfriend