Seccom Masada-sensei (or Masada) is a character found in Madotsuki's dreams. He is popular with fans. He is found by sleeping in any one of a number of beds found in the dream world, going down the staircase, walking past the elevator, and using either the Umbrella or Yuki-onna effect to extinguish the fire.
Description
His name is Sentimental Komuro Michael Sakamoto Dada-sensei (センチメンタル小室マイケル坂本だだ先生). Obviously quite a mouthful to pronounce, fans shortened it to Seccom Masada-sensei (the letter c being doubled probably to avoid trademark infringement of a security company of same name, which is famous in Japan).
He is found in a black-and-white spaceship, apparently the pilot. He stands next to a piano. The player is able to "speak" to him, yielding a beeping noise that some Mario players may find familiar.
Masada is the only character who walks backwards in the game, although the only way to see him walk is by activating the Knife effect.
Fan Speculation
Piano Teacher
Seccom Masada's usual interpretation is that he is Madotsuki's (possibly former) piano teacher.