The previous Kazz was similar to Kou. He was a beautiful child like Kou and was offered up as merchandise and "entertainment" for aristocrats. He never met his parents and was already in the orphanage when he was a newborn. Even so, the people in charge of the orphanage never took pity on him and only saw him as another mouth to feed. Fortunately or unfortunately, the abuse only started when the previous Kazz was 9 years old. By then, he already had a conscious like Kou, realizing this orphanage they call home was "hell." He has a very weak personality due to abuse and neglect from the orphanage, but when he met the Mukamis, he started to change little by little.
The leading cause of the previous Kazz being trapped in the closet was because of these little changes. He disobeyed someone with authority. That someone decided to punish Kazz, so he locked him up in the small closet room. He took the keys and hid it in his office. Luckily, Yuma saw this and told the others. They decided to rescue Kazz on the day of the escape. Why? First, Kazz disappearance in the closet room wouldn't attract that man's attention and second, they had to waste a day to find those stupid keys! However, they had to take a risk. They have to risk Kazz's life. Whether or not Kazz survive starvation with his poor body was up to him. The Mukamis are Kazz's family, but they had experienced things that made them ruthless.
The current Kazz received the previous Kazz's memories, but he decided not to dig deep into it or else he would be negatively affected. That's why he doesn't know where all those bruises and slashes come from...
The previous Kazz really died from starvation in that closet.