Gatou is one of the few wealthy people in the world. He runs a big shipping company, but he secretly sells illegal merchandise (like drugs) and uses ninjas and gangs to take over anything he needs. Gatou took control of the Wave Country's shipping industry and has a monopoly over all the business traffic. He's trying to get rid of Tazuna to stop the construction of the bridge, which will break his monopoly over the business traffic. He has two bodygaurds, one that likes to cut things up and one that has long hair. Gatou was responsible for killing Inari's father, Kaiza.
Zabuza is also known as the Devil of the Hidden Mist. He is a missing-nin of the Hidden Mist Village and was hired by Gatou to kill Tazuna. Zabuza is known as a devil, because as a child he killed hundreds of students without hesitation. He has a helper, I guess you could say, named Haku. He met him when he was a kid and wanted him only because Haku was of a special bloodline. Since then he has trained Haku and taught him many jutsu's. Here's a weird thing that Naruto noticed about him...he has no eyebrows!
Haku is the loyal companion of Zabuza. Sometimes I have trouble thinking of him as a guy, since it looks like he's wearing lipstick and he looks extra girly with his hair down. Anyway, his mother was from an 'advanced bloodline', which means that he has a super powerful jutsu that was passed down from each generation. People from advanced bloodlines were feared and were killed because of that. His mother was killed by his own father, who he then killed. Haku was all by himself afterwards, no longer wanted by the world. When Zabuza came along and wanted to use Haku for his bloodline, Haku felt like he was needed again. His only dream is to make Zabuza's dreams come true. For Zabuza, Haku would do most anything.
Inari is the grandson of Tazuna. He never knew his real father, so when Kaiza came along he thought of him as his father and they became very close to each other. However, Kaiza was ordered to execution by Gatou and Inari became rather depressed and lost hope. He went to his room a lot and cried about it. After he met Naruto, he yelled at him that he'd never know what it was like to have a bad life, and Naruto snapped at him (because he did have a bad life) and called Inari a sissy. Later, Kakashi went out to talk to Inari and made him realize that Naruto knew what he felt like and comforted him. After that, Inari decides to stop crying and he rallied the village against Gatou's men.