Alpha 02, otherwise known to his clone "brothers" as "Spar" was one of twelve original Jango Fett clones called the Null-Troopers.
Spar also become one of the few rare clones to retain the memories of Jango Fett and had avoided being reconditioned like his other brethen. As a result he became obsessed with rebuilding the Mandalorian Supercommando's to their former glory before they had been nearly wiped out by the Jedi.
Making his way to the Mandalorian homeworld of Mandalore, Spar combined the forces of Mandalore's police force along with the surviving members of Vizsla's Death Watch to create a group he called the Mandalorian Protectors. He also reinstated the ancient tradition of the Mandalorian leaders bestowing upon themselves titles. Thus, Alpha 02/Spar became known as Mandalore the Resurrector.
Spar's significance to Boba Fett comes from when Boba's daughter Ailyn Vel murdered Spar years later after she mistook him to be her father because of the similar armor he wore to her dad's. Ailyn murdered this false "Boba" because she blamed him for being responsible for the death of her mother, Sintas Vel years ago after Boba abandoned the both of them. After killing Spar Ailyn took his armor and later masqueraded as Boba herself.