penitent

Short definition from Wiktionary

Adjective

1. 1
   Feeling pain or sorrow on account of one's sins or offenses; feeling sincere guilt.
2. 2
   Doing penance.

Noun

1. 1
   One who repents of sin; one sorrowful on account of their transgressions.
2. 2
   One under church censure, but admitted to penance; one undergoing penance.
3. 3
   One under the direction of a confessor.