The Mandalorian who challenges Din Djarin in the covert is Paz Vizsla.
Paz Vizsla is a heavily armored warrior belonging to the Children of the Watch. He first clashes with Din in “Chapter 3: The Sin” after objecting to Din accepting Beskar from an Imperial source. The Armorer stops their knife fight, and the two later fight together when the covert is attacked by bounty hunters.
Paz challenges Din again in “Chapter 5: Return of the Mandalorian” from The Book of Boba Fett. After Din presents the Darksaber, Paz claims it as his birthright because the weapon was created by his ancestor, Tarre Vizsla, the first Mandalorian Jedi. Din defeats Paz in a duel, after which Paz declares him an apostate for removing his helmet.
Paz Vizsla is often confused with Pre Vizsla, a different character from The Clone Wars and Rebels, or Tarre Vizsla, the ancient Jedi who created the Darksaber. Bo-Katan Kryze is another prominent Mandalorian leader, but she does not challenge Din in that covert duel.