If it is the title of a priest, like Friar Tuck, you do. Otherwise, just to refer to a person by his role, friar is not capitalized.
Friar John -The only other Friar who appears in the story 8)
the friar... the friar...
Friar John
A friar is a male.
St. Francis of Assisi was a friar, not a deacon. He founded the Franciscan order, which emphasized poverty, humility, and simplicity in devotion to God.
He sends a friend and fellow friar, Friar John, with a letter.
Friar Tuck
Friar Laurence asks Friar John to deliver a letter to Romeo explaining the plan to fake Juliet's death.
Friar Lawrence married Romeo and Juliet in his cell.
Romeo asks friar Laurence to marry him and Juliet in secret
Romeo and Friar John are not related. Friar John is a Franciscan friar who was supposed to deliver an important letter from Friar Laurence to Romeo, but he was unable to do so due to being quarantined during a plague outbreak.
In "Yours truly," you capitalize the first letter of each word if you are using it as a complimentary close in a letter, as it is a formal sign-off.