相手にする (deal with/pair up)

A set expression meaning 'to deal with, pair up with, treat as a partner.' 相手 ('partner, opponent, counterpart') + にする ('make into') combines to form the idiom. Also has a negative sense: 相手にしない ('ignore, not take seriously') and a passive 相手にされる ('be taken seriously / engaged with') / 相手にされない ('be ignored, not taken seriously').