Kyouya is the only one talking about the blader's spirit. Hikaru and Tsubasa say they are going to battle Ryuuga, but they never talk about it. We don't even know if they believe in it. More importantly… Let's look closer at his quotes about the blader's spirit, in episode 49:

Kyouya: But I have the blader's spirit. This is how Leone and I fight and win.

Ryuuga: How nice for you two. Now I'm just gonna crush that spirit like a bug.

Kyouya: This is far from over. Go ahead and try but you'll never touch my blader's spirit.

And later

Dark Power: The only way to heal our wounds… The only way… to heal our wounds… We need blader's power to.

Kyouya: Okay, that's really nice. Go ahead, and try. But I'll never let you steal from me my blader's spirit. thinking Ginga, take a good look. This is my blader's spirit.

Kyouya doesn't only talk about his blader's spirit as something that'll help him win, he also says openly he'll prevent Ryuuga, L-Drago and the Dark Power to influence his blader's spirit.

I notice it because I search it to make this post… But Kyouya's spirit is the first one to help Ginga in episode 51. First as the shield preventing Ginga to fall

then as an added power

The Dark Power wasn't supposed to infect Kyouya.

First, Kyouya has all this arc with the Wolf Canyon that differentiate him from Hikaru and Tsubasa. And the Dark Power doesn't overwhelm him as he does with Hikaru, Tsubasa and Ryuuga.

And those aren't the only differences. Let's take a look at two other points showing the difference in Hikaru's, Tsubasa's and Kyouya's reaction, and consequently on Dark Power's influence on them.

Psychological influence

The Dark Power, and Ryuuga, uses a psychological ascendancy to win. Their power impress, or even scare their opponents. If it's not enough, however, Ryuuga doesn't hesitate to use words to make their opponents doubt about themselves.

Hikaru loses even before the duel's beginning.

"Why did I- Why did I think I could ever win for a moment… against a guy like that?"

-Hikaru, episode 41

Thought Tsubasa resists longer than Hikaru, using the strategy he planned to battle Ryuuga, he loses before the battle's end.

Ryuuga: What's wrong? If you think you can dodge it, give it a try. If you think you can stop it then go ahead and stop it. Why don't you?

Tsubasa: Don't quit Eagle.

Ryuuga: I congratulate you for being able to make me use some of my real strenght. As a reward, I'll take extra special care and stealing that power from you. (...) It's finished. You were reckless and foolish to even consider challenging me in a battle at all.

Ryuuga: So say hello to the pit of despair!

-episode 45

Tsubasa lets Ryuuga's words (and power) touch him without replying. At the end of their duel, he's entirely passive.

On the other hand, we have Kyouya. He is evenly matched with Ryuuga, driving him to give up the Dark Power for their duel and even snatching some kind of compliment from Ryuuga:

"You are no match for me. (…) However, I can't remember anyone taking so many of my direct hits without completely collapsing. I give you credit for that."

Kyouya fights Ryuuga as if he was any other blader (an interesting blader, not a horrible monster he has to defeat to protect the world). He returns every blow and taunt, without being impressed, and even less scared, by Ryuuga. When L-Drago's Dark Power intervenes, Kyouya isn't afraid…

Ryuuga: Ah what?

While Ryuuga is shocked by the Dark Power's apparition. This is the first time he shows such an expression.

Kyouya is destabilized, somewhat worried for Ryuuga, but he isn't afraid.

Kyouya: Ryuuga? What's wrong? (…) Yeah. Just when I thought it was getting good. Now the circus' act is becoming a bit much, huh? Hey Ryuuga?

He tries reaching out to Ryuuga but the Dark Power entirely overwhelms him. It soliloquizes about its need and Kyouya's answer is just:

"Okay, that's really nice. Go ahead, and try. But I'll never let you steal from me my blader's spirit. (then, thinking) Ginga, take a good look. This is my blader's spirit."

Kyouya stays in fighting mode until the end of their battle. The last sentence he says is adressed to Leone, "Shatter him! Let's use everything that we've got left!"

If Kyouya loses the duel, he isn't the one who loses against the Dark Power. Ryuuga is. The Dark Power controls him as a puppet at the end of the battle.

A symbolic detail

The duel between Kyouya and Ryuuga is one of the most important of Beyblade Metal Fusion. It is as decisive in the anime and the manga, though not for the same reason.

Beyblade Metal Fusion - episode 49

The whole point of Kyouya's and Ryuuga's battle is reminder of the blader’s spirit.

“But I have the blader’s spirit. This is how Leone and I fight and win!”

Kyouya encourages Ryuuga to give up the Dark Power's tricks and to fight as a true blader. He succeeds and, for a time, they have a duel spirit against spirit.

Watching their battle, Ginga realizes Ryuuga is a blader too. He didn't have this thought before, and is somewhat puzzled by it (we can't blame him since the past he shares with Ryuuga).

“Bladers spirits splashing until they are burned out. Well that is a true blader like Kyouya and even Ryuuga.”

He certainly wouldn't have understood it without Kyouya. Consequently, he wouldn't have tried to save Ryuuga from the Dark Power and everyone would have lost in the end.

"If I fight force with force, I cannot win. They save me in order to give me that message. Save... That's it! (...) I'm no gonna abandon a fellow blader. That's exactly what the Dark bey has been doing."

- Ginga Hagane, episode 51

Beyblade Metal Fusion, volume 4

In the manga, the duel between Kyouya and Ryuuga is fierce too. It isn’t about the blader’s spirit but about stopping Ryuuga no matter what. Kyouya succeeds to damage L-Drago (he’s the only blader who achieves this feat). He weakens L-Drago attacks, allowing Pegasus in the next fight to survive its special moves (it’s in black and white in the manga), and lowers its defence/stamina, so Pegasus' attacks are able to affect it.

To conclude

“I'm gonna win it. I have to, so I can battle Ginga who'll be waiting for me once I get past this.”

- Kyouya Tategami, episode 49

"Ryuuga! You're just one small step on my way to get to that final where Ginga waits for me, got it?!"

There is a slight difference in the french dub. There, Kyouya says: 

"Ryuuga! Tu n'es qu'une simple étape sur mon chemin. Je veux seulement atteindre la finale où Ginga m'attend moi, tu piges?!"

First difference, in the French dub, Kyouya uses two sentences. He begins by “You are a mere step on my way”. Mere seems even more disrespectful than one small since he implies Ryuuga is nothing.

The second sentence is “I only want to get to the final where Ginga waits for me, got it?!” Kyouya talks clearly about his goal, the one he thinks about since the begining of the episode, not giving a thought about Ryuuga and L-Drago’s Dark Power. He shows Ryuuga has no role into this story: it’s only about Ginga and him. It follows his thoughts before the duel, where he was only thinking about Ginga, and almost forgot he had to fight Ryuuga first.