I watched this movie long time ago. I almost forget everything now. LOL I'm sorry. But I do remember that It's the movie for cat-lovers. The cats were cute even though I prefered more the story of Kazama.
In my opinion, I could imagine the cats as Mitsuo's children. Mitsuo would be a bad parent. He put his own dreams on his cats. He wanted Kuro to be the boss despite its willing. Conversely, I think Mitsuo's brother was a good brother. He took care of his brother. For example, when Mitsuo knew that he couldn't do boxing anymore, he cheered him up and gave him money and when he decided to leave, he brought the cats for his brother. I think it was his intention all along to let cats be Mitsuo's friends. As a result, having cats with Mitsuo was good for him. It was the motivation for Mitsuo to go on living, working, and finding something meaningful for his own life.
Mitsuo always thought too little. He just wanted Kuro to follow his dream without considering the possible consequences. Moreover, he suddenly wanted to do something and then he started drawing manga just because his brother do it. He thought that it was easy for an amataur to win a prize. He didn't even do research or anything. That's why his life seemed to be miserable. However, at least, he didn't quit his job just to be a manga-writer. That was good.
Mitsuo was a little biased towards Kuro, wasn't he? I tried to rewatch the movie a bit and I found that he always took care of Kuro more than Chii.
P.s. I really respect Chii and Kuro. They were good at acting. I think it was not easy for the cats to act the way producer wanted.