There two answers to this.
The easier answer is: So what? Rudy didn't choose to get into this situation, he can't change it at will, he's trapped there in this life as a young boy. You guys told me last week that there'd come no good from Rudy telling anyone the truth about himself, so at that point you also have to accept that he's gonna live life as if he was that boy, Rudy. Whether he still has some ideas from his past 40yo-life or not doesn't matter, because, again: He didn't choose all of this. Do you all know the Tom Hanks movie "Big"? In that movie, an underage boy ends up having sex with an adult woman when he magically turns into an adult man's body over night. Is the woman a rapist for having sex with someone who looked like an adult, but clearly behaved like a kid? It's basically the reverse situation. The answer is the same: Nobody chose this, so accept reality and move on. Rudy will forever be a boy/man around Selphie's age (assuming she's not already 100 yo, considering her elf-dna. Although her human parent is alive, so probably not). No matter how much time passes. When Sylphie is 20, Rudy will be, like, 55. Will that be okay? When Sylphie is 40, Rudy will be, like, 75. That okay? The point: It's a new life. That's literally the point of an isekai story. And acknowledging that makes it acceptable imo.
As a sidenote: I'd like to challenge you guys' "he shouldn't tell anybody about his former life, it wouldn't do him any good". Have you NEVER felt guilty about something and, even knowing you'd probably get punished, just had to tell someone about it? I find it rather shocking that I'm supposed to be the only one who's like that. I remember when I was in kindergarten and I wasted the sticky tape roll. Kindergarten teachers asked everyone who did it. Nobody said a thing, me neither. As I was hiding on the playground outside, I felt miserable and eventually approached one of the kindergarten teachers and cryingly admitted that it was me. I receiver a couple stern words, but I also felt so much better afterwards. Lying to people feels bad, having to keep lying feels worse. If I were Rudy, I'd absolutely tell someone. Maybe I'd tell Sylphie, both to get it off my chest and because she's nice, and also to give her the choice of removing me from her life in case she's scared/creeped out.
Now, the more complicated answer. Warning: If you cannot bear philosophical discussions, don't read on. The following does NOT mean that I'm in favor of "grooming" or child sex. It's a hypothetical scenario, just like this anime, and I'll be writing about hypothetical approaches that I'd never try in real life for a myriad of reasons. So keep your accusations to yourself.
Anyway, you accuse Rudy for the "molding her to my liking" and yes, it sounds bad at first glimpse. It's certainly perverted, yeah. But I have though about this many times before (because unfortunately the whole pedo-shit is a recurring topic when you're an adult fan of anime and japanese video games. sigh. Just yesterday I saw someone call fans of Xenoblade Chronicles 2 "pedos" for liking the character designs of the game ...) and honestly, the term "grooming" is total bullshit. The issue that "grooming" tries to indicate is that evil adults trick children into acts of sex. That's the short version of it, the popular one, I think. I take two issues with that: First, what when a "good" adult approaches a child and doesn't do any "tricks" and once the child reaches Age of Consent, the two enter a relationship? Everybody is happy, nobody is hurt ... but society condemns the older person and pushes everyone to misery. That's the one issue, a very hypothetical one, again, thinking about it philosophically. Nobody is hurt, everybody is happy, but somehow it's still evil? Eh. And there's my second issue: How is "grooming" any different to regular "hitting on women"?! The whole "tricking" is the very same. You see a woman you find attractive, now you do your best to "convince" her to accept you and let you fuck her. That's the key difference: "Convince" being used instead of "trick". The real world meaning is the very same. You trick a woman, and a bad person convinces a child. Conceptually, from the man's perspective, grooming and hitting on women are the same. There is one more difference, yes, and that is the mental capability of a woman vs a child. The biggest issue I take with that argument is that NOT ALL WOMEN ARE THE SAME. And thought further, NOT ALL PEOPLE ARE THE SAME. I've been taking note about this topic for a book I plan to write. Basically, I refuse the societal distinction between "child" and "adult". Don't misunderstand, there are some differences and the laws we have a pretty good for the most part. What I hate, however, is how we treat children like raw eggs, but the moment they turn 18 (or 21 or whatever your cutoff age is), we give a literal crap about them. But biologically, there is no such cutoff age. And there never will be. People have different experiences in life, different mental makeup. And no matter what, all adults are always the child they once were. I often get really sad when I think about that, looking at some random 50yo guy or 45yo woman, how cold they'be become, how they just "function" to continue to go to work and earn money, maybe have children, but that's it. No passion, no real joy beyond "this is what I must do, because society has forced me to become that way". I often just wanna hug these older people and pat their heads and tell them "It's alright", but, lol, I'd probably be pushed back, called a creeper and have the police on my ass ^^ "Age" is something that relates to everyone of us, and yet our society handles it sooo badly. The worst one being the distinction between "adult" and "child", beyond some good laws. To go back to Rudy: When he says "mold her to my liking", it's really two things: First, a random degenerate weeb's bullshit thinking. Considering he worships Roxy's panties, he might have thought that simply for an internal meme. Second, everyone does that to some degree. Yeah, I can already hear you say "No, I don't. When I'm attracted to a woman I want her as she is". But that's bs. Before you even approach her, you chose her because what you saw was "to your liking". That's where it begins. Then you start talking, it's generally pleasant. But you find out she smokes, and you think "I'll help her quit smoking". Then you spend more time with each other and you find out she voted Trump, so you think "fuck, I'll try my best to make her realize that Trump and the Republicans are evil". And then, after marrying her, she asks you which of 3 hairstyles you'd prefer on her, and you choose the one "to your liking". Point being: "Molding someone to your liking" is normal, we do this everytime we interact with people and want more interaction with them. We want more because of the parts we like, but the parts we don't like we hope to "improve". And realistically, what's Rudy gonna do to Sylphie? He already accepted that she's probably gonna be better at magic than him soon. He helped her escape being bullied and ostracized by others. Now he spends time with her and we haven't seen him do anything out of the ordinary. So, what do you expect when you complain about his line "mold her to my liking"? That he convinces her of some sexual practices? Sure, once she's old enough. When I had sex with a woman who never had given a bj (she was 30, in-before dumb accusations ...), I had to ease her in on the idea, too, before she eventually did it. That's normal for anything that someone hasn't done before. If DarthEnderX loves to have anal sex, I doubt his gf was like "whatever". When Ryll first asked his gf to pee on him, I'm sure her first reaction was "eww!". So I ask again: What do you think will Rudy do bad in regards to "mold her to my liking"?
Phew. That was longer than intended. I hope you actually read and understand that, but I don't havem much hope for that happening and instead expect some "WTF" and "omg" and "you're defending grooming, dude". No, I'm not. As I clearly explained above. These are mostly philosophical thought experiments and if you don't have the mental capability to think abstractly, then fault lies with you. With utmost clarity: I condemn anyone who grooms a real child or otherwise engages romantically with a real child.
So you're incapable of understanding "surprising someone in her sleep" as anything but rape? Wow. I guess English isn't your first language.