Why he's not the main character in undisputed 2. Why Iceman? and in undisputed 3. It's a Russian. In undisputed 4(If happens). The main character is the one who defeated by the russian in undisputed 3. In this movies the one who losses become a main character in the next movie.
shareIt is a way of reusing interesting characters. One literary rule for martial arts stories (and yes boxing does count as a martial art) is you have a totally overconfident "villain" who is arrogant and narcissistic and thinks he is invincible. The "hero" of the story becomes the FIRST person to defeat said villain to prove that the villain is not unbeatable. Once the villain is beaten he is used up and normally cannot be reused. For example the early 90s flick KICKBOXER 2 was boring because Tong Po had been beaten in the original thus proving that he is not invincible. Similarly you could not have a very interesting spinoff of Rocky 4 where a different hero takes on Ivan Drago because Drago has already been proven to not be invincible.
However there is no literary rule requiring the HERO of a story to be undefeated or to seem invincible. The hero of the story is SUPPOSED to be the underdog. Look how much Daniel got beat up in Karate kid before finally winning and Rocky Balboa was not undefeated either. So if you have a villain in a martial arts movie (as long as you dont make him evil beyond redemption like Chong Li in Bloodsport or Tong Po in Kickboxer or Brakus from Best of the Best 2) and he is defeated by the hero of the story you can still reuse him in a sequel or spinoff as long as he is the hero and not the villain.
For example Johnny Laurence could have been used as the hero in a spinoff of Karate Kid and Clubber Lang could have been the good guy in a spinoff of Rocky 4. The writers of the Undisputed movies figured this out and learned to get more bang for their buck by using each villain twice, its hard to find good actors who can also do martial arts so its more economical to use each one twice.
Who cares? The movies wasn't about him, but of George Chambers who gets transferred to a Russian prison in Undisputed II.
Undisputed III was more than likely made to capatalize on Adkins popularity as the character Boyka. In my opinion, he was more interesting a character than Chambers was.