。
She always sides with the underdog.
Literal
She [topic-は] always weak-people [to-に] sides-with.
味方する (みかたする, 'to side with, to support') takes に for the person or group being supported. 弱者 (じゃくしゃ, 'the weak, underdogs') is a formal word for those in a disadvantaged position. The pattern describes a consistent moral stance — she reliably champions the disadvantaged.