BTCPay Server's own foundation announcement describes the project as having started in August 2017 as a one-man project by Nicolas Dorier. The repository dates from 18 August 2017 and the software is MIT licensed. His GitHub profile lists Tokyo.
The design decision that defines the project is the one he made at the start: BTCPay Server is non-custodial and self-hosted, so payments go directly to the merchant's own wallet and the software never needs the merchant's private keys in order to receive. That removes the processor from the transaction rather than replacing it with a more agreeable processor, and it is why the project charges no fees. There is nobody positioned to charge them.
The BTCPay Server Foundation, announced in September 2019 to collect corporate support, records Dorier as founder of the project and states that he holds two votes for the purpose of breaking a tie. The announcement is explicit that the Foundation does not control development or claim ownership of the software.
Only his public professional role and published work are recorded here.