Since he has always wanted a little brother, he has taken a liking to Ryoma. Recently, however, it seems like he has lost his favorite Ryoma over to Ryoga. As a result a burning sense of rivalry with Ryoga has developed.
Tokugawa has been playing tennis since he was 5 years old and spent many days abroad training hoping to become a professional.
Despite being a second year high school student, he has a Large Motorcycle license.
In Japan, 16-year-olds can legally obtain a Small Motorcycle license, but only 18-year-olds or older can acquire a Large Motorcycle license.
With strength that 2nd stringers can't keep up with, Tokugawa is the center of the crowd of veteran tennis players.
Tokugawa was staying in Paris when he was called into the U-17 camp.
Tokugawa is the only 2nd stringer high schooler scheduled to play against one of the top 10 1st Stringers.
It can be speculated that he is rich like Atobe from all his abroad training trips and his favorite hobby.
Tokugawa has similar traits to Yukimura:
Both were undefeated until they met Byodoin (Tokugawa), and Echizen (Yukimura);
Both have played tennis since a very young age: 5 (Tokugawa) and 4 (Yukimura);
Both are known to be quite strong: Tokugawa is known to be the the strongest 2nd stringer while Yukimura was the only Middle Schooler chosen to be involved in the inital challenge to the 1st String.
Tokugawa also has a similar role as Tezuka:
Both would only observe Echizen and only later (after a match) would they open up to Echizen and teach him many things. Since Tezuka left, Tokugawa has filled in the role of "top rival" for Ryoma to want to defeat and is a mentor to Ryoma.
Both have similar personalities. They both are always serious and barely smile.
They both have the initials TK (Tezuka Kunimtisu; Tokugawa Kazuya)
Even though Tokugawa and Byodoin do not get along, they do have a few similarities:
They both lost in their first year of the camp;
They both were chosen for the sake capture mission (Tokugawa likely since he knew about it in the current year);
They both have trained relentlessly to defeat the one person who has defeated them in an official match: Oni (Byodoin), and Byodoin (Tokugawa);
They both have been travelling abroad for tennis training;
They both want(ed) to become the No. 1 All Japan Representative (Byodoin previous year and currently holds, while current year for Tokugawa);
They both like tea (Tokugawa likes tea while Byodoin likes Green Tea);
If Tokugawa's first name Kazuya, was jumbled around, it would spell out "Yakuza".
Tokugawa is the most featured high schooler in New Prince of Tennis merchandise.
Tokugawa is the only tennis player Byodoin has encountered twice. Everyone else Byodoin has defeated were either traumatized or never touched tennis again.
Tokugawa and Yamato are the only known high schooler who has balanced stats.
Interestingly enough, they are both 2nd year high schoolers.
Even though Tokugawa respects Irie and Oni and has a rivalry with Byodoin, he managed to do something none of them succeeded: picking up a girl, in the Pick-Up Contest.
He is also the only non-3rd year high schooler to do so.
He thought the contest would be fun.
Tokugawa is the only Japanese student in their 2nd year of schooling who participates in the Pre-World Cup Exhibition Match. Fuji, Duke, Irie, Atobe, and Yuikimura are all 3rd years.