While many Super Smash Bros. Ultimate fans expected that a Tekken rep was coming, nobody expected Kazuya. 

Kazuya isn't usually considered one of the most popular fighters in Tekken. He was definitely not predicted by Smash players. The Super Smash Bros. community was more focused on Jin, Akuma, or Heihachi. 

But Masahiro Sakurai recently explained why Kazuya was chosen to represent the classic fighting game series over Heihachi. Sakurai said that they "wouldn't capture the true essence of Tekken" like Kazuya could. 

In the Kazuya presentation, Sakurai said that Tekken and Super Smash Bros. were both fighting games — but completely different. Tekken is about spacing from the other fighter while Smash is about positioning on the map. And Smash is a lot faster, with many characters able to attack five times faster (or more) than Tekken fighters. 

Mario's jab in Smash is two frames while Kazuya's jab in Tekken is 10 frames. But Sakurai didn't want to speed Kazuya up just for the sake of it. He wanted to keep the feel of Tekken. How? Introducing Tekken's Devil Gene to Smash. 

This is a comeback mechanic for Kazuya, something other fighters have in Smash (including Sephiroth and Lucario). It makes his attacks stronger and gives him an extra special one-time attack. Essentially, Sakurai noted, this made him feel more like a Super Smash Bros. fighter thanks to the smash attacks and special moves. 

Explained Sakurai: "It's something Kazuya's mother, Kazumi, carried. As the name implies, it allows one to draw out the power of the devil. Kazuya inherited the Devil Gene. [...] Heihachi does not have it. This is the main reason why we didn't go with Heihachi as a fighter." 

Heihachi was almost in Super Smash Bros. Ultimate

A recent interview with Sakurai revealed that Kazuya wasn't a top choice when it came to repping Tekken in Ultimate.

Heihachi was considered at first but the character seemed too difficult to put in the game. His movement would be tough, Sakurai said, because his moveset in Tekken wasn't easy to translate in Smash effectively. For example, Heihachi and other fighters have a very low jump. But Kazuya can make up for that by using his Devil Form.