"one who or that which softens," in any sense, c. 1600, agent noun from soften.
Entries linking to softener
soften v.
late 14c., softenen, transitive, "mitigate, diminish" sorrow, etc., from soft (adj.) + -en (1). The meaning "make physically soft" is from c. 1400; the intransitive sense of "to become (physically) soft or softer" is attested from early 15c. The earlier verb was soft (v.) "make soft," attested from early 13c. but now obsolete, perhaps last used in Spenser. To soften up (transitive) in the military sense of "weaken defenses" is from 1940. Related: Softened; softening.