Is This "Unhindered" An Adjective? Adverb?

I agree with you. Unhindered- looks very much like an adverb. The way I was taught to tell apart an adverb from an adjective was, if "it " answers how, it is an adverb. In this case, how did he enter the shower? Unhindered. Also if the word is placed at the end of a sentence, it is more likely an adverb. e.g. The airplane landed safely (undamaged) despite its engine suffered a hit by a flock of birds.

voynichAt that time, was the culprit able to enter the shower unhindered?

grammarfreakThe way I was taught to tell apart an adverb from an adjective was, if "it " answers how, it is an adverb. In this case, how did he enter the shower? Unhindered. Also if the word is placed at the end of a sentence, it is more likely an adverb.

That rule is not 100% reliable; for example if "He felt miserable", then "miserable" is an answer to "How did he feel?", although "miserable" is obviously not an adverb.

However, this example is possibly slightly different. For me, "Unhindered" is an answer to "How did he enter the shower?" only in the same loose and not-exactly-logical sense that "Satisfied" is an answer to "How did he leave?" if we are told "He left satisfied". You wouldn't call "satisfied" an adverb, right?

GPY"He felt miserable"

OK,interesting! maybe I used the wrong example. We know all the sense words, such as feel, sound, look, taste and etc. end with an adjective. e.g. it tastes / sounds really GOOD!

GPY "How did he leave?" if we are told "He left satisfied".

If I am not mistaken, we have plenty of situations where an adjective ends a sentence but its semantic role is that of an adverb. For example: He came home drunk, He finished last, she woke up feeling hungry, or I was just thinking out loud. By the same token, this is what I see. If we replace "satisfied" with " feeling cheated", which is an adverbial modifying answering to "feeling cheated", I think then it plays an adverbial role, even though it is defined as adjective.

GPYYou wouldn't call "satisfied" an adverb, right?

With this interpretation, despite its apparent definition, I do see "satisfied " possessing certain adverbial property.
I must note. My analytical approach could be wrong. I am not a linguist, just someone who loves English, and has studied it for many many years.

voynich was the culprit able to enter the shower unhindered?

One view has it that unhindered in this sentence is a subject-oriented secondary predication. Such predications share the qualities of adjectives and adverbs.

CJ

I was hoping you would drop in CJ. Thanks for your input. 

CalifJimSuch predications share the qualities of adjectives and adverbs.

CalifJim subject-oriented secondary predication

Is this an official name for such structure?

grammarfreakHe came home drunk, He finished last, she woke up feeling hungry, or I was just thinking out loud.

For me, "drunk" is an adjective there. I find "last" hard to judge. "hungry" is an adjective. "out loud" is an adverb phrase.

GPYFor me, "drunk" is an adjective there. I find "last" hard to judge. "hungry" is an adjective. "out loud" is an adverb phrase.

I think you are missing the point. I am not so concerned about their individual labeling, but their semantic role in the sentence. I think CJ's explanation makes a lot of sense, and satisfies both sides. If you are out-loud " as an adverbial phrase, then the adverbial property still holes true with " he came home drunk". That was my point.