Via American Catholic:
Marcellinus and Peter were prominent enough in the memory of Church to be included among the saints of the Roman Canon. Mention of their names is optional in our present Eucharistic Prayer I.Read more about these saints here.
Marcellinus was a priest and Peter was an exorcist, that is, someone authorized by the Church to deal with cases of demonic possession. They were beheaded during the persecution of Diocletian. Pope Damasus wrote an epitaph apparently based on the report of their executioner, and Constantine erected a basilica over the crypt in which they were buried in Rome. Numerous legends sprang from an early account of their death.
Why are these men included in our Eucharistic prayer, and given their own feast day, in spite of the fact that almost nothing is known about them? Probably because the Church respects its collective memory. They once sent an impulse of encouragement through the whole Church. They made the ultimate step of faith.
"The Church has always believed that the apostles, and Christ's martyrs who had given the supreme witness of faith and charity by the shedding of their blood, are quite closely joined with us in Christ" (Dogmatic Constitution on the Church, 50).