I re-read chapter 50, 60, 72, and 75~79. there was nothing to suggest he is the informer, but also nothing to suggest he isn't. he was always loyally following yasumoto's reckless/illegal orders with only minor complaints
chapter 79 also shows kubo & shino getting fed up with yasumoto https://mangadex.org/chapter/1147323/9 , so in retrospect having informant help eliminate yasumoto makes sense.