Professor Raymond Knowby sought Father Allard's help with destroying the Deadites after inadvertently releasing them. After Knowby's death, Father Allard presumably stayed at the Church, and continued preaching biding his time...
When Ash returned and his hand released the Deadites once again, Father Allard helps Ash after Ash saves him from a demonic tree.
A Devil in Disguise
Father Allard is later revealed to be Evil Ash, and adopted the guise of the Father as to have Ash regain the remaining Necronomicon pages.
Father Allard is a very cowardly man, letting Ash do all the "dirty work" while he stays in the church yard with the Book, waiting for Ash to collect the the remaining pages. Ash often calls him "Padre" (as he does with all Priests he encounters in any of the games) and makes fun of his hat.