You could use: "lamentably"
It sort of means the same thing.
Look it up.
Reluctantly has four syllables.
He agreed very Reluctantly. Do not show me your reluctantly or I will fire you.
No, "reluctantly" is not a preposition. It is an adverb that describes the manner in which an action is being done.
reluctant
willingly; eagerly
willingly Eagerly
She reluctantly agreed to go to the party, even though she had other plans.
Reluctantly, she jumped into the cold water.
reluctantly
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Reluctantly is an adverb.
He was reluctant to provide an example. They gave up the puppies reluctantly.