Saint Achilleus was an early Christian martyr, venerated in the Catholic Church as a soldier and a steadfast follower of Christ. Alongside Saint Nereus, he served in the Roman imperial army, reportedly under Emperor Diocletian in the late 3rd to early 4th century. According to tradition, both Achilleus and Nereus converted to Christianity, abandoning their military roles out of loyalty to their new faith and refusing to participate in the persecution of Christians. For this defiance, they were martyred and later buried in the catacombs of Domitilla in Rome. 

Achilleus is celebrated for his courageous faith, which led him to embrace martyrdom rather than compromise his beliefs. His feast day, shared with Saint Nereus, is observed on May 12 in the Catholic liturgical calendar.