First, we have a character: "The guard." Guards separate what they protect from the people they protect it from. So the guard isn't the person who has a problem here. Whoever is noticing the guard very likely has a problem. Guards feature in stories of prison escapes and heists. When we see a guard in a story, we expect someone to need to disable or sneak past them. The word "guard" already sets the story up for action.
Next, we have "looked." "Looked" here means "appeared to be" and when we use it that way, it almost always implies "but actually wasn't." Jang could have written "only looked" and she didn't need to. She dropped the unnecessary word "only" and the sentence is briefer and has more impact for it.
The fourth word is "entirely." You know this one. "Entirely" protests too much. The real world is mostly or partly or nearly whatever it is – when someone says "entirely" they are exaggerating for effect or implying that they mean "not at all" instead. Both "only" and "entirely" are adverbs (words which describe how something acts or modify descriptions and often end in "-ly") and to use two adverbs in this short sentence would have sounded absurd. The one Jang chose heightens our awareness that how the guard looks isn't how they are.
The final word, taking all the extra emphasis of last position, is "human." I love seeing "human" there. "Looked" and "entirely" have already set up the final word to be untrue. "Human" arrives as a world-shifting zinger.
The sentence now contains the meaning, "The guard wasn't human." Our world doesn't feature guards that only look human. This marks the story's genre as fantasy or possibly science fiction. With five words, Jang prepared us for action in another world. What fun!
The whole book was a lot of fun. It has a fast pace, and a richly seen, fresh setting. I'm glad I saw this on a Big Idea post on Whatever and I'm glad I read it. I will look for more from Jadie Jang.