Kozo appears to be middle-aged, presumably around fifty or so. He has shaggy graying hair with a large fringe and sideburns, and has a mustache and thick eyebrows. He always dresses formally, even at home.
As a normal human, Kozo has no supernatural powers, but is very skilled with a rifle.
Kozo is a kind man, though perhaps a bit gruff at times. It's also hinted that he was a bit of a fighter in his younger years. He deeply cares for his wife and especially his children, who are the last things he has on his mind before his unfortunate passing.
Kozo is first seen a few days after Akira fused with the mighty demon Amon to become Devilman. He is sitting in his home with his wife, children, and Akira. He sits and watches as Miki and Akira play and flirt with each other, while Akiko making a passing comment on how Akira now reminds her of Kozo when he was younger. Later on that night, he and his wife are absorbed into the walls of their home by the demon Agwel. He is freed when Agwel and Sirene are defeated.
Kozo is seen later on during Lord Zennon's mighty reveal to the world about the existence of demons and the imminent attack on the human world. Akira tells Kozo to head inside and protect his family; Kozo says he'll gather his hunting rifle and lock the doors. During the attack, Kozo tries to comfort Miki who was worrying about Akira, when he hears a window smash upstairs. He goes to investigate, with Miki following behind, and sees the horrific sight of the demon Loscead making his way down the stairs. He fires at the approaching demon, hitting it in its head; however it launches snakes from its body which attack them both. Luckily they are saved by the sudden arrival of Dosu-Roku and Meriken-Jo, who proceed to kill the snakes.
After the Demon Busters are initiated by the Japanese government, Kozo is called to join his neighbors in expelling the family of a boy who had been accused of being a demon. Kozo does not want to do this anymore, but Akiko pleads with him to go, explaining that not participating will raise suspicion that he and his family are demons. Kozo reluctantly agrees and joins the witch hunt.
Later in the Makimura house, Dosu-Roku, Miki, Tare, and Masa talk about how the demon attack wasn't all bad. However, Kozo relays upon them the truth that the demon attack has served to turn humanity against each other; people are killing those who they once considered friends, and the only way for people to shield themselves from demon accusations is to accuse other people of being demons. Kozo contemplates that, rather than another attack from the demons, the weakness of the human mind may well be what leads humankind to its destruction.
Akira then comes in and Dosu-Roku asks if he found Ryo; he says no. Wanting to cheer Akira up, Miki turns on the television for him. Shockingly, a news report comes on with an interview with Ryo. In the interview, Ryo begins to slander Akira before showing a video tape of his first transformation into Devilman. As the tape plays and shows Akira's slow and hideous transformation, everyone panics. Akira tries to hold Miki, but Kozo aims his rifle and tells him to get out. After Akira explains himself, Kozo says that, whether Akira is still himself or not, he had done his family no harm, and tells him to flee before the Demon Busters come for him. Akira, amazed that Kozo and the others believe him, smiles and flees.
Unfortunately, not long after this, the Demon Busters arrive. Kozo claims that Akira escaped after seeing Ryo's interview. The captain shakes Kozo and scolds him for letting Akira escape. Kozo asks the captain how he could expect a human to let a demon escape. This leads the captain to doubt that Kozo is really a human, and arrests both him and Akiko.
Later Akira runs into Dosu-Roku who tells him the Miki's parents had both been arrested and were now being tortured. Akira tells him to go and gather all the Devilmen he knew, and with that, the entire Devilman army is gathered together as it flies over to destroy the Anti-Demon Corps HQ. In the basement rooms of the destroyed building, he comes across the torture chambers, where he finds Akiko's body. Akira continues to explore the building, where he eventually hears someone faintly call his name. He turns to see Kozo chained at the wrists in a large, spiked torture machine. Kozo has enough strength to call out his children's names before coughing up blood and dying. Akira finds the three men responsible as they beg for their lives, but he ignores their pleas and engulfs them in flames.