Yes. It's rare- Nightmares are birthed into the world for a specific purpose, and their natural temperaments orient them towards vicious aggression. But though their ostensible purpose is to destroy life, to do so effectively they need to take on the aspects of living beings. Bodies need food, so they will feel hunger. And to sustain itself, a life form needs to do more than eat, so they have a range of feelings, including emotions. To be effectively autonomous beings, it's inherently necessary that they can process their own thoughts and make decisions.
So though nightmares generally exist to kill Dreamkeepers and the vast majority of them conform to that task, it is possible for some to disagree and formulate their own objectives.
But a disobedient Nightmare is not dealt with kindly when their compatriots gain access to them. And once they die, their spirit will return to the Nightmare realm before being reborn, so there's ultimately no escape from their peers.
23 Who are your personal favorite dreamkeepers and why?
David: Mine would have to be Whip. I just like how he's a bit of a detached observer, a touch sardonic, yet very keyed in to his own desires. Liz's favorite Dreamkeeper has yet to appear in the series.
23 Probably super plot vaulty but here i go: Can a dreamkeeper ascend into a nightmare or a spirit and how do they do it?
To be honest I'm not sure what this question is asking. I'll just ramble a bit and hopefully answer it. In Dreamkeepers lore, characters have souls, and they can also have a body to run around in. Dreamkeepers have a body and share the same soul with their human counterpart. Nightmares (when they manifest one in the Dreamworld) have a body. In Volume 4 we discover an unconfirmed possibility that Spirits can be summoned into and trapped within a body. So if that turns out to be what really happened, then it'd be canon that souls can be implanted into bodies that were not originally theirs.
21 During Volume 4 when Mace asked the group to take a break, did he ask because he realized Bast was limping or because he was actually tired?
Mace did notice Bast was limping, and that's why he called for a break. I think he realized that bringing attention to Bast's injury would be counterproductive, considering how prickly Bast can be- he might deny it and refuse to stop. I figured in light of Bast's insane escape from the Neons in Volume 3, a delayed-action injury would be a necessary bare minimum consequence to depict.
16 How big can Ryuu Nekos get and is there any sort of special qualities they acquire at their full sizes?
Full grown ryuu-nekos commonly grow to become a hundred to two hundred feet long. If they are bred and fed for that purpose, they can get to that size within 10-20 years, but normally it'll take about a century for a wild neko to attain that size.
They tend to eat less and less frequently as they grow, so when they reach a hundred feet or so, they may stop eating altogether save the occasional big meal. That means that their growth ceases, which is why this tends to be their maximum size.
Nekos are said to grow more intelligent as they age, so large ones tend to be very perceptive.
There are legends that one neko of each type continued to grow, becoming a thousand foot behemoth, and that they rest hidden- underneath rivers, winding below mountain ranges, buried beneath rambling jungles.