There is no single inventor of DeFi, since they are all built on top of Ethereum by different developers, and Ethereum was invented by Vitalik Buterin.
Currently, the largest and first DeFi app is MakerDAO, which was founded by Rune Christensen.
Renowned venture capital firm Andreeson Horowitz has made multimillion-dollar investments in both Compound and MakerDAO, the flagship of the current DeFi ecosystem.