！！？
What?! You're taking her side again?!
Literal
What [よ]! Again that child [gen-の] shoulder [obj-を] hold intention!?
肩を持つ (literally 'to hold someone's shoulder') is the fixed Japanese idiom for 'to side with / take someone's side.' 気 here means 'intention / state of mind' — ~気 attached to a verb phrase (the preceding clause) asks 'do you intend to X?' 何よ is a feminine exasperated 'what?!' The whole utterance is short, sharp, and jealous-sounding — typical of a romantic or peer-group conflict.