Do Mother Brain or Ridley Appear in the Game?
No. Neither character is in the game, but they are both mentioned in passing by Samus and various flashbacks.
Are X Parasites the same as Metroids?
No. X Parasites can replicate you and steal your memories. All Metroids do is suck the energy from whatever they touch.
Who Is the Main Antagonist?
That depends on your take. There are two main antagonists. One is the SA-X, which is essentially your clone, and the other is the large Metroid.
Where Does This Game Fall in the Series?
It is the fourth game in the Metroid series. Fusion was released just before Metroid: Prime.
Is There a Famous Timed Ending Sequence?
Of course. It wouldn't be a Metroid game if there wasn't the famous timed ended.