Sara probably isn't the kind of girl who would hold a grudge.
Literal
Sara [topic-は] resent-like girl is-not [conjecture-だろう] but.
恨むような marks a hypothetical quality: 'the type who would resent.' ~ではないだろうが concedes a point while leaving room for doubt: 'probably not, but...' The trailing が suggests the speaker has more to say. セーラ is a Japanese rendering of the English name Sara/Sarah.