That's because he's part of the Six-Awns-Equilibrium or whatever you want to call it (六芒均衡/Rokuboukinkou)
Being the most elite ARKS members would mean he's too busy to be seen there :/
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Ok... you know that the maps are generated at random... and i already told you that i didn't took a screenshot... i didn't thought about it...
What's the point in asking me to give you a screenshot of the area? is impossible right now!! XD
It happened in V.Cave, during a Breeahda extermination code... the NPC had his name write in white, no HP bar under his name and it was a custom NPC, he had that new scarf that some players wears, red and spiky hair, wired lances... it wasn't that Huey from the 6 pillars, that's for sure, or else i won't ask you about him.
I know that the information i'm giving you are nearly useless... it's just a thing... maybe this strange thing will happen to some of you someday or maybe to someone of you already did... just let me know if someone may knows more about this strange event XD
Let's make this clear: It is completely normal for an NPC to spawn as part of a code:attack (most codes have different variants, especially protect). You can tell if they're an npc because they have that same green/yellow icon next to their name as if they were partners, as well as the cut-in asking for help whenever the code initiates.
It is also completely normal to not see his/her HP bar. When you say "under their name", do you mean the name directly displayed above him/her (with or without actually displaying HP bars for players in a quest), or the actual e-trial brief info under the radar map?
Either way, it's fine. The npc technically isn't in your party AND is an npc, so you wouldn't be able to see the hp bar above him/her anyway, and if you couldn't see his/her hp in the e-trial info, it's probably a code change over (so it doesn't matter how many times they die XD)
As for your second scenario, you didn't specify that there were no other players around, so another player probably just came in with that partner character when the code initiated.
Then a code like this initiates, the NPC searches for the MP leader - that means, the player that has been in the area for the longest time.
That aside, I know of a better easter egg. This is for PSZ fans. Doesn't ring when creshida gives you this Client Order abotu completing Tundra Arks member Rescue in less than X minutes with a gunslash?
And when all of sudden you realsie you're wielding Axion?