The list of all Bitcointalk users' usernames is very big but you only want to know the first account, person who joined this forum.
loyce.club/usernames/usernames.txt

Look at very first names.
1: admin
2: does not exist
3: satoshi
4: sirius

First topics https://bitcointalk.org/first_topics/
The first topic is created by the #1 account (admin): https://bitcointalk.org/first_topics/1.pdf

Probably it is the founder, Satoshi Nakamoto.