n. One who fosters; one who is somehow designated to care for and nurture someone.
the GNU version of the Collaborative International Dictionary of English
n. One who, or that which, fosters.
The Century Dictionary and Cyclopedia
n. A nurse; one who feeds and nourishes in the place of a, parent; hence, one who or that which promotes or sustains: as, a fosterer of rebellion; intemperance is a fosterer of crime.
Word Usage
"Through this marriage Joseph was not a mere fosterer, nor was he an adoptive father of Jesus."