Sorry if this theory was stated before,but i have read a theory that the boss might be Heizo Hattori , he first appeared in volume 10 , his interpretation was made due to the fact that:
--Heizo is really smart and cunning -judging by the file where he hit his own son to solve the case- and keeps a low profile somehow
--His position as he is the highest ranking officer at the Osaka Prefectural Police Headquarters
--He doesn't want his son to be a detective,maybe because he doesn't want him to get involve in catching the BO
--his name, which happens to be the same as Heizo Hasegawa is the main protagonist of "Onihei HankachÅ" , he was an ALCOHOL ADDICT , he was also called the "villain" -just like vermouth once described him- he used to solve difficult crimes by hiring criminals as informants , so basically he hire criminals to help him achieve an ethical goal - solving the crime- in an unethical way which i found it so similar to the boss , as we all know the BO goal is to create an non aging drug which i believe is a really good thing but the use unethical ways to achieve their goal by hiring criminals to help him.
Finally, A photograph of Heizo Hasegawa appeared at the "Gosho Aoyoma's Mystery Library" page from volume 15
