Probably, and he's probably going to find out there was actually no reason for him to be holding back in the first place. So far, the story has been told from a limited omniscient point of view, but we've seen no reason why he should need to be so cautious, and plenty for why she shouldn't need to be.
- He destroyed Felice's root, she is no longer a demon lord, and has no connection to ever having been one
- He either has a cheat skill no one knows anything about (potentially, including himself), or simply has no power they could rip away from him
- They already know he has a talking cat named "Felice" since they brought him in, and him having a similar cat would check out with before his summoning/return (if they cared to investigate it)
- Other heroes have familiars or powers related to monster taming (seen in this chapter)
- He is strong enough to defeat a demon lord capable of destroying 3000+ worlds and defeating countless heroes in the process, as well as wielding a weapon that even said demon lord hesitated to use. They could not defeat him even if they tried.
What does he have to fear? As far as we know, nothing. I had previously asked others if I was missing anything, but all they could give was "the goddesses weren't involved his his summoning" and "he doesn't have any cheat powers" which are both non-issues (see points 2 and 5 above) based on the way an individual character understands things (those characters may have flawed knowledge, they are not omniscient), and still doesn't explain how they could make the connection. Heck, we don't even know they'd be hostile if they
did find out, everyone is just assuming they would.
I did offer some explanation before, that you either missed or glossed over, but here is a more detailed explanation. At the bottom is a list of things he would potentially have to fear, that you seem to fall under the "am I missing anything" category.
Since a person's "Root" is their soul, and Felice still seems to have her soul, her root doesn't appear to be destroyed. (Though perhaps she has her "ego" and her Soul/Root were destroyed, but I think it was the "ego" of her hacked decision making that he wanted to destroy, to restore her soul to what it was.) Even in the early chapters, we see him wanting to get rid of the artificial shackles on her Root, not necessarily destroy the Root itself. It also appears that part of why she lost her powers was helping protect him from Lavanquin, which he was not ready to use at that point (as evidence by her "if I hadn't helped you in the end. We'd have both disappeared together with this world" statement, which would require her to have had some powers after he did the initial strike, in order for her to help him). What we see her say she lost was: "My powers, my shackles, my destiny, my curse." Of note, she seemed to have a Root before the "Demon Lord" shackles that gave her her "destiny" and "curse". She says she is free from her "Binding Root", but that may also mean "Root Binding" (I'm not a Japanese language expert), and seems to tie into her "shackles" being gone. So either she has a whole new Root of some sort, or her Root has been restored to something more like it was before being artificially modified.
So the goddesses might be able to recognize her base Root, and want Revenge. (But if so, they might also be able to recognize it is no longer shackled, which might lead some to deem her no longer a threat, while others still would want to kill her.) I don't consider this to be the biggest concern, however.
More to the point (and something I had mentioned to you), though, is that she is REGAINING her powers. We've been told this multiple times. We also see that, without her powers, she can't maintain humanoid form, but she does like using her humanoid form. At least some goddesses would likely recognize her humanoid form (something I hadn't mentioned before). The fact she is regaining her powers, though, means there is a connection to her past that still exists in her and is getting stronger, and could, at some point, be noticed.
Another thing I mentioned, is that they said the school is designed to recognize people like Felice, and nip them in the bud before their powers grow too strong. So if they see her humanoid form and recognize her, they would likely try to do something about it (something she says would be "worse than death"). Quite possibly, it would be to seal her away in isolation again (and her isolation might be the reason she brought somebody over to kill her, though it could be because she was worried she was about to break out and go on another rampage).
As for Kyouya's strength, we don't know if he could defeat all of the other goddesses or not. They couldn't destroy Felice, but they were capable of sealing her for some period of time (likely at least 10s of thousands of years, since 10,000 years pass in her world for each one in ours). She also, obviously, did not defeat all of them. But it may be that there was some sort of stalemate, or that they couldn't kill her faster than she could devour worlds to gain strength.
It is easier to fight when you aren't also having to protect attacks aimed at others, too. Felice is not nearly strong enough to oppose the goddesses, and she is the one he wants to protect. So, while he might eventually be able to defeat them, they might be able to kill Felice (or somebody else he ends up close to, as they know he has compassion for others). They have shown the ability to teleport people into pocket spaces (as they do for different training), so it might be possible for them to transport Felice when he is just too far away to notice in time, as just one way they could harm her. They might also be capable of sealing him and Felice away for an extended period of time.
He also doesn't want to fight them. He wants a peaceful life with Felice, and fighting with a bunch of goddesses (and never knowing when they might try to strike, so having to be constantly vigilant) would get in the way of that. Especially since they have connections with many (if not all) governments of the world (and almost certainly all of the most powerful ones).
So, what does showing his strength do, before Felice has fully recovered her powers? It causes them to pay more and more attention to him, and if they do that, they are more likely to see Felice in her humanoid form, and not just her cat form, and if even one goddess recognizes her appearance, they have trouble, regardless of if they can identify her Root or not, especially if they see that she is weak, but regaining her strength.
It is true that we don't know for sure that they would react with hostility if they knew she was no longer the mindless destroyer of worlds they once knew, but Kyouya doesn't see that as worth taking the risk. This makes a lot of since, seeing as how one of their stated goals is to "nip such a bad thing in the bud before it grows up", and that fact that she is now back to being "in the bud" as it were, and the fact that some of the goddesses definitely act in a petty, cruel, vindictive, and evil manner, means that at least some would likely show hostility. So if he wants a peaceful, normal life on earth with Felice, especially once she has the power to constantly be in humanoid form, having the goddesses pay him as little attention as possible is ideal. (Especially since another stated goal of the school is to keep an eye on heroes that have the power to destroy the world.)
What does he have to fear? Many things.
1. That if he tried to fend off all of them, they might still be able to harm an incredibly weakened Felice.
2. That the goddesses might manage to seal both Kyouya and Felice in another world, isolating them (AGAIN) for thousands of years, and preventing him from showing her a peaceful life.
3. That the goddesses have all grown stronger over time, and raised up more heroes, that might mean they now could win a fight against Kyouya and Felice. (You might say that it was only 3 years of earth time, but maybe they can travel to other worlds that also have slowed time, or can make pocket dimensions that slow time, or travel to another world that has slowed time and make a pocket dimension for slowed time on top of that. So he can't bank on them only having "3 years of growth".)
4. That even if he can win against them, and can keep from being sealed in another world, that he and Felice would have to constantly be vigilant for attacks to come, and thus it would disrupt the normal, peaceful life they are aiming for on earth.
5. That the only way to get the goddesses to leave him alone would be to destroy at least a significant portion of them, which would likely lead to conflict with multiple governments on earth, and could also put multiple other worlds at risk of destruction from Demon Lords that the goddesses are helping keep in check.
6. That even if he could win, it might mean killing heroes that honestly thought they were doing the right thing, and he doesn't want to kill "good people" (and even prefers not to kill people acting in an evil manner, like the rogue hero).
7. That there might be something more powerful than the goddesses (like whoever might have artificially changed Felice's Root) that might start paying attention if he gets into a conflict with the goddesses (or that the goddesses could contact for help, if they thought he was on their side, and didn't know he was the reason Felice went on the rampage in the first place).
The question is not about how likely these scenarios are, but if he thinks they have even some possibility of happening, if Felice is discovered. Because even if you think the odds aren't high, if he thinks they are possible at all, and would get in the way of the peaceful life he told Felice he would give her, he likely wouldn't want to take the risk if he thought he could avoid it by laying low.
