His straightforward character is trusted by many people.
生一本な態度で交渉に臨んだ。
He approached the negotiations with an honest and sincere attitude.
純粋 means 'pure' in a general sense, including physical purity. 生一本 focuses on moral purity and straightforwardness of character.
Compound of 生 (pure, unadulterated) and 一本 (one straight line), suggesting a single-minded, unswerving purity of character.