I think it started when I was around 16. I had started to watch foreign movies and in one of these movies there was an actor called Takeshi Kaneshiro. I liked him, he was cool and was especially funny in "The Returner". It also helped that he voiced Samanosuke Akechi in The Onimusha Games which btw were totally awesome (still hoping a live action movie will happen).
Anyway, I started to go by Takeshi_K since Takeshi_Kaneshiro was too long, and Takeshi was most of the time taken. Takeshi_K sounded cool and sleek. Most of the time when I saw that Takeshi had been taken I checked out the forum members. I remember getting kind if pissed whenever I saw that those who had registered under Takeshi at the different forums weren't active at all! How uncool, at least use your damn account when you take my nickname! I've had a few others, though Takeshi_K is the one I used the most.
I like the fact that I was able to get Takeshi on BakaBT because this place is quite big.