Should Robotnik be called Eggman or Robotnik

You can refer to him as either. His full title is Dr. Ivo "Eggman" Robotnik.

Story line goes, his real name is Dr. Ivo Robotnik, later got his nick name "Eggman" from Sonic.

Character creation goes, SEGA of America named him Dr. Ivo Robotnik, and SEGA of Japan named him Dr. Eggman.