faithfully
adv.doing something in a loyal way or exactly as you were told. You use this when someone stays true to a person, a promise, or a set of facts.
adv.in a loyal and constant manner; with strict adherence to facts, a promise, or an original version.
“He served the company faithfully for thirty years.”
“The film adaptation follows the original novel faithfully, keeping every major character and plot point intact.”
“Despite the changing political climate, the secretary faithfully executed the duties of her office, ensuring that every document was archived according to the established protocol.”
Typically follows the verb it modifies or sits at the end of the clause.
From Middle English feithfulli, equivalent to faithful + -ly.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.