R

Yup

Nope. Final hint:

[spoiler]It's a weapon.[/spoiler]

Nope. It can be used unlimited (but it has limit) in a single turn and also can be used in the following turns.

Nope

Nope, its not a minion.

Not even close. This card is unlimited in other way. Also you understand that you found answer when name of the card will fit last two lines.

[spoiler]Okay. It is difficult to give a hint without spoiling the riddle. The possibilities of this card are limited, but also in some extent are not limited. The last two lines refer to the name of the card. The first two - to its effect.[/spoiler]

And they just print the same spell but also add rush to a minion...

I have a little unlimited power.
For this reason I'm not a cruel.
But if you spend with me hour
I will make you look like a fool.

[card]Fizzy Elemental[/card]

[card]Coilfang Warlord[/card]

The first three lines are about the secret keyword. And the last line is counterspell itself.

Yup

On the right track. The last line is the key.

[spoiler]This cards can be into the battlefield and be kinda invisible but its another keyword.[/spoiler]

It's not a minion.

Nope

Nope

We have kind of invisibility.
But it's not our main ability.
We have a lot of usability.
I can cancel your activity.

[card]Soul Split[/card]