By the way, for those of you who have read my character sheet and have knowledge on the respective franchises that I used for him, do I need to redo the sheet to be even slightly more realistic? I'm more than willing to if it turns out to be an issue.
Okay, I wanted to respond to you about your character some time ago, but overall, I agree with what everyone already said. That said, I'll pick apart what I think are the problems...
-His backstory is a mess. Ignoring certain things (like the abandoning in Japan and Lina being Nanoha's mother), it just screams Sue. I mean, at age five he's already an assassin (dragging Nanoha into it...for some reason), helps in Abstergo's downfall (age five), gets involved in the plot of Nanoha (this is the most plausible part), next part I'm not familiar with, but gaining better abilities while lacking mobility. Which, apparently, also healed him. Then, after that is the inverse, Nanoha in HP. How they got invited there despite living in a far away country without HP backstory like Harry, I dunno, it's still one of the two more plausible parts. Another mix of familiar and unfamiliar elements, though I don't know what happens at the end. “Shot everything to Hell”? And then more mixed plots, now with StrikerS, and without much clarification.
Yeah...no.
-His stats...you put on the same scale as a Servant. Aside from recommending you use a scale of his own species rather than Servants, Servant scale is at minimum ten times human maximum. Which means your character is on par with a Servant. And some of those stats are downright ridiculous. I mean, A+++? Flying at the speed of light? Also, how can someone be an A on Servant scale, and E- on human simultaneously (as I've explained earlier)? Probably got those switched around. You don't give any stat for Mag. Ability, that's a no. The description just makes him sound even more Sueish. By the way, magic and strength are separate, so you should have written the D+ (which is still way too strong).
Actually...
-The descriptions. All of them give the impression of him being even far more overpowered.
Joking: “Seriously, mix Red Staff magical ability with Blue Staff magic abilities, and what do you get? Massive Overkill!”
Serious: “Harry Potter as raised by Lina Inverse, who's Nanoha's birth mother in this scenario. Magical abilities are something that come to him, not the other way around... and that's before he learns about his Magic Circuits”
“Handyman of the Year (that last one's because he's a pro at fixing things that get broken, including Devices. Ask Nanoha and Fate if you don't believe me)”
See what I mean? I'll just leave the “Show, don't tell” advice here.
Also, he has no weakness? No personality flaws that could be exploited? Doesn't he have some power restriction? Anything? At the very least write “None” rather than...
“any attempt to get to him would require overpowering his sister... good luck with that.”
That reads more like a taunt than anything. Refer to my point above.
Hell, not only that, but his “likes” implies he has Nanoha characters as a harem. Don't...please don't.
To be brutally honest...it hurt to read through it. I mean you no offense...but you seriously need to turn this down a few notches. Several notches.
I'm honestly not sure how that impression came across from it. I've not made him out to be particularly powerful, he's just vaguely competent and learning to be better.
As for his training, though, it's not so much they've been pushing him as it is he's been pushing himself, and they've just supported him in that.
Didn't Shirou practically die/not-quite-die because of pushing himself too much? I'd think they'd try to avoid him pushing himself, rather than support him on that.
Anyhow, if you want some advice on that, well...maybe turning down his abilities a notch could help. It won't mean he's helpless, it's just that having that much skill at his age just seems implausible, specially given the circumstances.