I encourage you to look at chapter 23, "Monster," in the book, for the very first descriptions of the Bird (or Corporal Watanabe), as they seem to sum him up the most directly. In his first interactions with Zamperini, he strikes the POW directly on the head, twice, for not looking him in the eye. The sentence following is perfect:
This man, thought Tinker, is a psychopath (232).
More background information is provided for him in the pages that follow, including a brief description of his childhood and entrance into the service. Here, it is first established how deeply patriotic Watanabe was, and how his generation was raised in a way that seemed to desensitize them to violence.
Throughout the imprisonment, the Bird only grows more and more sadistic in his desire to break Zamperini's spirit, and description of his actions are probably the most telling of his character throughout these chapters.
I also encourage you to re-read chapter 34 "The Shimmering Girl," which is about Zamperini's arrival at home. His first breakdown occurs in front of his family when his mother wants to play for him the record of his broadcast in captivity. Within seconds he is yelling to turn it off and smash it. Next,
Louie fell silent, shivering. His family stared at him in horror.
Louie walked upstairs and lay down on his old bed. When he finally drifted off, the Bird followed him into his dreams (334)
The effects of the Bird's physically, mentally, and emotionally torturous treatment of Zamperini and others would follow (and plague) many of them for the rest of their lives.
Finally, some more personal insight is given into the Bird's character at the very end of the Epilogue. In 1997, a CBS reporter was able to find him and pursue a few lines of an interview out of him in Tokyo. In it, the Bird was asked, "Zamperini and the other prisoners remember you, in particular, being the most brutal of all the guards. How do you explain that?" (396). His answer includes that he was treating prisoners "strictly as enemies of Japan" and considered "beating and kicking...unavoidable." This again speaks to the Bird's deep sense of patriotism and training in violence and harsh punishment. He walked proudly, as if he had nothing to be ashamed of and nothing to hide. He never apologized for his actions.