Kija had waited for Hiryuu (or his heir) for a long, long time. He had been born for the purpose of protecting Hiryuu's heir, and would die if it meant he could do that.
He wasn't sure how he had expected the Red Dragon to be when he came. Perhaps a strong, amazing creature that awed Kija and inspired intense loyalty inside of him.
Maybe he expected him to be kind but distant, gentle but commanding and incredibly strong in battle.
One thing Kija knew was that he had arrogantly expected that the Red Dragon would require only Kija's strength, and depend on him and nobody else.
He wasn't quite sure what he expected the Red Dragon to be like, but Yona certainly wasn't it.
Sure, she was gentle, but she was far from distant, constantly conversing with him and aiming to be his equal (although he always thought she would be higher than him, for she was every bit as amazing as expected).
She was gentle, but didn't quite command. Sure, she gave them orders, but it was more of an aura, where Kija instantly knew what she wanted without needing to be told.
And though Yona strived to be, and was becoming, she had a long time until she was strong in battle.
Nevertheless, Yona was definitely the Red Dragon.
What else could make Kija feel so happy when he so much as thought of her, after all?