Uhhh isn't Yuriy a friend too?
Hmm for Manga, final Bey is Dranzer Metal Flame. However, after the final match against Takao, he switches back to Dranzer MS. Personally, I think it's a good choice, considering that Dranzer MF is quite pathetic uncustomized.
Final attack for anime... quite controversal. I don't know what's the english dub, but in the Japanese anime, when he used it against Brooklyn, he said something like "This is... my love."
There was an arguement over this on another board XD. Right now, I think of his final attack as "Ai Da". Just that it doesn't really sound like an attack name, XD.
Just put the Manga final attack, End of Flames. You know, it's quite cool that he also learnt Takao's killer technique, Ultimate Storm.
[spoiler alert]
After seeing Takao's power at the world championships semi-finals, he learnt his technique first, then developed End of Flames as a counter (since he would have understood Takao's weakness). The night before the final match, both of them went to the junkyard where they had their first battle, Ultimate Dragoon VS Ultimate Frostic Dranzer (isn't it amusing that those were called "Ultimate"? Please compare with Dragoon and Dranzer MS.). Kai used Ultimate Storm, and it had exactly the same strength as Takao's Ultimate storm. Following that, he used End of Flames. Dragoon was unable to counter, but did not get defeated. Kai talked about not having anywhere to return to, having betrayed BOTH BBA and Neoborg to gain the power to beat Takao. Takao tells him that he is always welcome to join them, because they are all friends.
The next day, they appear with Dragoon MF and Dranzer MF. The manga stops at this point; the chapter after that shows what happens many years later, when they're all adults.