Resplendently is the adverb: in a resplendent way — shining, richly, so that the look itself is the point. “She was resplendently dressed” means the clothes were not just fine; they glowed or dazzled. You will mostly meet this in writing. In speech, people are more likely to say beautifully, magnificently, or just so brightly. It describes appearance, not character. Though actually, you would not usually say someone behaved resplendently unless you were making a small joke about how they looked while they did it. Save it for what the eye can see.