Sentimental Komuro Michael Sakamoto Dada-sensei (センチメンタル小室マイケル坂本ダダ先生) (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
The Yume Nikki thread on 2ch gave him the name, "Sentimental Komuro Maikeru (Michael) Sakamoto Dada-sensei" or "Sekomu Masada-sensei" (セコムマサダ先生) for short, as a combination of "Sentimental," Komuro Tetsuya, Michael Jackson, Sakamoto Ryuuichi, and Dada. "Seccom" is a result of an English-langage simplification of "Sekomu."
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. Interacting with him during the Mars event (as the ship is crashing) will show that the noise is higher-pitched (he may be panicking here, so that's self-explanatory), and when the ship lands on Mars, the noise is lower-pitched (possibly showing a bitter atmosphere, or Masada may be dying).
Masada is one of only two characters who walk backwards in the game (the other one being Dave Spector), although the only way to see him walk this way is by activating the Knife effect. He glides forwards when the Cat effect is used.
Fan Speculation
Piano Teacher
Seccom Masada's usual interpretation is that he is Madotsuki's (possibly former) piano teacher. It has also been interpreted that Masada might have been a drug user, and the crashing of his UFO was meant to represent his 'crash' (Overdose, possibly his death as well).
There has been speculation that Madotsuki had lived and slept in Masada's flat outside the dream world (represented as an UFO in her dream) for some period of time, this could suggest that Masada is an adult that she trusted before she became a shut in for some reason. Their relationship seems to be platonic, it could be that Madotsuki had a brief feelings for Masada before, but didn't know her feelings towards him (in which he speaks an alien language). However the siren could represent police sirens on their way to arrest Masada as the authority suspect that he is having a nonconsent relationship with Madotsuki due to her being a young teenager.
Also the UFO where Masada lives in is simplistic and spartan, with only the big piano looking control and a table with two chairs only, could suggest that Masada is a struggling pianist who earns side income from private piano lessons.
Death
Many fans speculate Masada was a drug user, and may have died from a "crash", or overdose. This is represented when the alien ship crashes on the alien planet.
Another rumor is that he may have died in a train accident. This is shown when going down into underground Mars; the flights of stairs there are similar to those of a subway station. It is also underground, like many subway stations. At the bottom is a large pile of what seems to be train wreckage, with a figure (Mars-san) crying next to it. Mars-san could represent the people who mourned over Masada's death, and the wreckage itself could point to death by a train wreck. The fact that sad music plays during it only boosts this. Also interacting with Mars-san results in a small 3 note piano song playing. This could be one of the mourners telling what happened, a story about Masada, or just mourning.