After it was approved by users in some Brazilian forums, I'm going to post it here:

In (on, at? I'm always confused) her researches, Ciel discovers a strange force, an evil energy source. Trying to use it, she causes such an explosion that destroys her lab completely. Understanding its power, she decides that this source could not get into wrong hands. She then tells Cerveau about the situation. He constructs an infantoid where that force, called Kōriki (which means high force), was sealed.

That infantoid was named Areon and entered to the Resistance. He was more powerful and agile than the three Resistance soldiers together. He was forbidden to go too far from the Resistance, and he didn't understand. However, two years later, he was going to talk to Ciel when he heard her talking with Cerveau about the Kōriki, understanding then his supernatural force and that prohibition (forbidness? How do I spell it?).

Irate, Areon ran away from the Resistance and began to wander around the world. Meanwhile, a criminal organization (for lack of a better word) known as Vintage begins attacking various places of the world. A week after Areon's flee, he is back to the Resistance..

The attacks are going worse, then Ciel contacts Zero, that heads for clues about Vintage.

Firstly, an infantoid is an adolescent-looking robot.
What do you think about this idea? My purpose is to turn it in a game. What can be better? Please, tell me your opinion.
Don't ask how Vintage came out, because it's a spoiler.

So a resercher found an evil source of power that they could not control. Cerveau builds a robot so it can control the Kōriki.
The robot is sent to the resistance to fight with them, but it must stay with them at all time.

Well first, how did the infantoid knew about the Kōriki?
That makes no sence. Please re-tell that part.

It seems like a good idea for a game in my opinion

No, because the Kōriki was discovered by Ciel, which is part of the Resistance (didn't you ever play Megaman Zero?), and it can't control the force. It was built to seal the force, not to control it. AND the robot won't fight the Resistance (since it was made by them).

The robot knew about the Kōriki from Ciel and Cerveau. As I described, it (let me call "he") was going to talk to Ciel, but he finds her talking to Cerveau about his supernatural force. Irate, he began to wander around the world.

dude first off megaman ROCKS! both the original and X. That said, your idea for Areon that you call an Infantoid, could totally work. infact I wish that I would of thought up the idea for an infantoid LOL. Because an infantoid has the potential for numerous applications in a megaman game. If you want to take this infantoid design to the next level... you should definetely consider the possibility of having the robot age over the course of the game or even several installments if you decide to draw it out that long .

I say "age" because this infantoid could be the 1st of its kind where organic material has been introduced into a minature version of the original x bot design. i like the story so far... if you are any good at robot master design you could start drawing rough drafts of some of these, (or facesets so you dont show off too much), and profiles for each.

I know pratically nothing about the megaman zero storyline but I'm assuming it takes place after the events of megaman x. Anywho i'm interested in it, i'd love to hear some more!

Too bad I'm horrible at Reploid designing.
In fact, the infantoid won't survive too much. Actually, it will be killed at the end of the game. Why? It's secret (an ill-keeped secret). If you want to know, I'm going send you a private message.