普段はとてもおとなしいナイスガイだが、理性のタガがはずれると人格が豹変する二重人格。

、。

He's normally a very quiet nice guy, but when his rational restraints come off, his personality does a complete one-eighty — a split personality.

Literal

Usually [topic-は] very quiet nice-guy [copula-だが], reason [possessive-の] restraint [subject-が] comes-off-[if-と] personality [subject-が] completely-changes dual-personality.

ナイスガイ is the English loan. 理性のタガがはずれる ('the hoops of reason come off') is a vivid metaphor — タガ (箍) are the metal hoops that hold a barrel together, so when they come off, everything falls apart. 人格が豹変する uses 豹変 (literally 'leopard-change,' from a Chinese idiom about a leopard's spots) meaning a sudden, dramatic transformation. 二重人格 ('dual personality') is the clinical term. Rich in idiom.