Penitent comes from the Latin word paenitere, which means to repent. It can be either noun or adjective. Though it's not restricted to them, penitent often pops up in religious contexts, particularly as a noun, as in, "The penitent threw herself at the bishop's feet and begged forgiveness for her sins."

What is penitence in the Catholic Church?

a penitential discipline imposed by church authority. a sacrament, as in the Roman Catholic Church, consisting in a confession of sin, made with sorrow and with the intention of amendment, followed by the forgiveness of the sin.

Where does penance come from?

The word penance derives from Old French and Latin paenitentia, both of which derive from the same root meaning repentance, the desire to be forgiven (in English see contrition).