Sirius, a.k.a. Martti Malmi, was the second Bitcoin developer after Satoshi and the founder of the Bitcoin Forum. He's not an active developer anymore, but continues to manage the domain names bitcointalk.org and bitcoin.org.

Reference : https://en.bitcoin.it/wiki/Sirius

If I combine everything in this thread and bitcoin wiki then it is obvious that "Cobra" is "Sirius"