What pronoun is softly?

The Word 'softly' is not a pronoun. The Word 'softly' is an adverb, a Word that modifies a verb, an adjective, or another adverb.

Example: Mother sang softly to the baby.

A pronoun is a Word that takes the place of a noun in a sentence.

Example: Mother picked up the baby and sang softly to him. ( the pronoun 'him' takes the place of the noun 'baby' in the second part of the sentence)