MB is the AI of Mother Brain, Samus Aran's long time nemesis. The AI took on the form of a human, dubbed Melissa Bergman, fitting in seemingly with the initials MB. Melissa Bergman lied about her last name to Madeline Bergman making her feel like she was her daughter. This ended up fitting in like 'Mother Brain' and 'Melissa Bergman'.
 Metroid: Other M=
Not knowing what, or who MB was, Samus took caution. MB told Samus much needed information about the vents that were occurring on the Bottle Ship and what the Galactic Federation was getting into. MB told Samus that she was Madeline, not Melissa, and so Samus thought MB was one of the good people.
After meeting the true Madeline Bergman later on, Samus' suspicion about Melissa rises. After a long talk with the real Madeline, Samus is informed completely of what MB really is and how she became what she is.
After this, Samus turns around to set out to find MB, but when she does, Melissa is standing right there.
A fight ensues, and MB has a group of Desbrachians to back her up. You are forced into first-person view, and must defend Madeline from the creatures. In order for the fight to end, Mellisa must be targeted. When this happens, a cutscene plays, and the game will end after a while.
MB may or may not return in the future.