I've been going for the Yurei (Never Spotted) and Oni (Kill Everyone) medals on my first playthrough. What follows in the spoiler text below would definitely be considered a spoiler for Chapter 9 to a greater or lesser extent (depending on your opinion of such things) as my description will involve specifics of what happens in Chapter 9 from a story point of view.
That being said, I feel like the "event" I'm talking about in Chapter 9 shouldn't be a spoiler for you if you've been even vaguely watching the story cutscenes during Chapters 7 and 8, as the game tells you, in no uncertain terms, that you're going to have to do the "event" at some point! The only thing the cutscenes don't tell you is that it
occurs in Chapter 9.
If you're in any way concerned about spoilers for this game's minimal (but admittedly engaging) story content, don't click the spoiler text! Just don't do it! You won't be happy if you do!
Anyway, I think that's enough priming for even the most story-centric, spoiler-avoiding players so consider yourself warned if you open the spoiler text below!!!
*** Spoiler - click to reveal ***Start Chapter 9, pickup the only Scroll in the starting area and proceed to the second and final area of the Chapter (the door in front of you when you spawn in). Once you're in this area, it's only the random NPC guards that count against the Yurei medal (Hikaru and the exploding lights don't seem matter). Here's my exact run though I fully expect this level of detail is more than most people want...
I took out all of the random NPC guards without being spotted by any of them, and before interacting with any of the exploding lights or being seen by Hikaru and still got the Yurei medal at the end.
How do I know this for sure? Simple, Hikaru spotted me 3 times before I landed the killing blow but none of these sightings occurred until after I'd taken out of all the random NPC guards and before this Chapter I hadn't unlocked the Troll achievement...
The only way this set of circumstances could come about is if Hikaru counts as a guard for the Troll achievement, but not for the Yurei medal. I can be certain the exploding lights have no effect on the Yurei medal as they certainly haven't in previous Chapters and didn't in this one either.
Anyway, happy hunting my fellow Aragmi shadow people!