Here is my personal theory on Kakuzu: Kakuzu is a hybrid ninja, put together from spare parts of other ninjas, over the course of time. As we will find out, his eyes are mechanical, replacing his damages sharingan eyes, which he lost as a young man. Yes, that's right, Kakuzu is Obito. Since most of Obito's body was crushed and damaged beyond repair, he was later found and reconstructed with various ninja parts. His super strength stems from his arms, which were donated from a decendent of Kisame's shark clan, who are naturally retardedly strong. Which is further enhanced by a secret bloodline, which changes the bodie's composition to super dense materials. The "sewing" jutsu was later added by the ninjas who found him, in order to keep his body together. The threads themselves are made of solid state chakra, formed from a bloodline which is secreted by his stomach, also added in by that clan. His obsession with money stems from his unique ability to assimilate body parts. He purchases better parts on the black market, and makes himself stronger. After the accident, Obito has lost all memory and the clan that found him, gave him the new name of Kakuzu. So, there you have it. Kakuzu = Obito.