I know it is never expressed in the series, but could it be that TK has given us clues on his death? In the 2nd Ova (Hell's Kitchen; Ep. 15 SUB) he says something that really puts me on hold. In the midst of yet another fantastic sacrifice, he states,: ["Right now, she's waiting at the airport lobby."] All the while saving his dear comrade from being set ablaze by the choppers' missiles. Moments later he burns up from the flames in a dramatic display and temporarily "dies." Thus, ending that epic moment. In an earlier episode (around the beginning of the series) he also proclaims: Hurry up! (after that, he said: "You can make it! Oh, run to her and hug her tight!" in Japanese). Though, this even being related to the other quote is unknown. 

Now here is my theory! TK must have travelled between Japan and America in his actual LIFE. He is most likely an American who lived in Japan for sometime. Thus, explaining why he knows more Japanese than he does English despite his frequent outbursts of English and as to why he understands his fellow teammates in the SSS (as most of them speak Japanese). Overall, I am not too keen on these particular specifics. 

...But based on the clues I have gathered (not only from the quotes or characteristics/descriptions, but from the trivia section), I am almost certain of how he died. How about this?: TK was on an ordinary travel to see a loved one. Possibly a girlfriend or fiancee. She, of course, was in a separate country at the time, so dear TK had to travel by plane. Now whether he was going to Japan or the USA is unclear. Yet, he had boarded an airline to see his beloved. Before the plane could reach the destination, it must have gone through some sort of difficulties. Ambushed, hijacked,...Meh, let's go with crashed. All I can conclude is that TK must have died on that flight and never made it to see his loved one. As she ["is waiting at the airport lobby."] He understands how important a loved one is and how quickly life will fly by. Proven when he says, ["You can make it! Oh, run to her and hug her tight!"]...because he never got the chance to make it to the airport. 

That's what I have for you. It was just odd to hear that one quote in the situation. It really made me think. Of course, TK is still a mystery and we may never know the truth. He could be just spouting out random lyrics again...Still, I want to stand firm on this theory. Anyway, I'd like to hear some feedback and opinions? What do you guys think? Also! Check out that OVA! Hell's Kitchen ( @ around 16 minutes is TK's dramatic sacrifice).