At a Glance: Kaiba
In Kaiba, people can use technology to physically store their memories on computer chips. In theory, they can live forever if their memories are always put in a new body when the old one dies. Also, some privileged people can buy other people's good memories for their own use. It's a chaotic mess of different bodies and memories floating around in a black market kind of system.
Our protagonist is on a quest for his memories, for an understanding of his true self. People without means are basically refuse, however, and since he lacks memories, he's poor, essentially a body that's been thrown away. Him and all the main characters introduced are often on the wrong side of the law.
There's a lot of planet-hopping as Kaiba travels around in search of any clues about his past. He meets people, trades bodies around a few times, and tries to help others with their problems. At a certain point, it's not just about the self-discovery journey of the main character, but about the mysteries and injustices of the society the story is set in.
It's weird for this anime to be so deep and profound and yet also so goofy in art style at the same time, kind of like Kill La Kill, but without as much emphasis on action. It has a melancholy feel to it. Some of the animation is just plain strange. Visually, it seems to recall old-school anime like Astro Boy, but the story lacks the innocence of shows of that time. While the main character is rather innocent, he lives in a world full of secrets, manipulation, and corruption.
Kaiba's main character having a hole in his chest seems to symbolize the dehumanized, painful world in which the story takes place. People have false memories, lost memories, broken bodies, scrap bodies, replacement bodies. In short, people are fragmented, incomplete. They all have a hole in them in some way. Kaiba's journey is not just about healing himself, but about healing society.