When I read this I think of
I am however not too familiar with all the required background, so please excuse if I cannot provide line by line answers to all the individual hints. But since no one took a shot at this so far, I dare to post this guesswork as a try.
I'll post what few I can match:
Hiding a big smile
is the demeanor he is typically described with.
"Rubeus" in this pseudo pig latin style of that story line certainly looks reddish to me. The latin word would be rubinus. "Rubeus" is supposed to mean red and to be related to alchemy, especially in contrast with the white Albus Dumbledore.
fond of dragons
he hatched his own dragon from an egg, called it Norbert
though Charizard ain't among my companions;
Yep, it's Hogwarts, not Pokemon
Sometimes I wish I could think before I speak
He gets into many of his troubles by trustingly talking first and later finding out that he shouldn't have.
my purity and ingenuity are what make me weak;
This one doesn't fit at all to me. As far as I recall, he is not pure, but a half-breed, causing much discomfort from both sides, and I do not recall him ever being accused of being too ingenious.
lighting god, fraud;
Also no clue. The fraud that comes in mind was Quirinius Quirrel in disguise, but I have no connection to him being a lighting god (or, if that's a typo, a lightning god) - OP cleared this one up to be lost in translation. It's supposed to be Fluffy, which is named Thor in OP's translation, his big, menacingly looking dog, who is a frightened Wiener by heart (that's the fraud).
a brother, far bigger than me, lucky him, you see, for he is complete;
could be Grawp, although this is only his half-brother, to be precise, but he is a full pure-bread giant, so far bigger and in this sense complete in a way Hagrid isn't.
I always befriend all of the creatures doesn't matter how hideous;
That's what he does all the time, Buckbeak, Aragog, Thestrals, etc.
He's also professor for the Care of Magical Creatures, and he gives his courses with passion.
For one I took the blame and had lotta trouble
If the "one" is referring back to creatures, I'm guessing Buckbeak here. Hagrid tried to take all the blame for Buckbeak's actions to save him.
all 'cause of a guy, man was he subtle!
well, that should be the death-blow to my guess. Nothing in or near Harry Potter is even remotely subtle. Guess I was wrong all along.
OK, besides my ranting, this could be Draco Malfoy (the one dragon hagrid is not fond of), who basically tried to exploit the Buckbeak situation to get Hagrid expelled, and he was going at it as subtle as they come in this story.
Or, if this does not refer to taking the blame, this could be Harry Potter himself, as Hagrid was sworn to protect Harry and got into many a trouble for that. Although I wouldn't get the sublte part then.