妹子 正直这个意思有很多单词啊 不是每个名词都有形容词形式的……

Im not sure there is one. If there is, I certainly cant think what it is off the top of my head. You would normally use it as a noun rather than an adjective. He / She has integrity. Depending on exactly what you are trying to say, you could use possible other adjectives trustworthy, honest, loyal etc

Integral?