The Onions protect them from the predators on their planet while they sleep. As you can see each night, the Onions fly above the surface of the planet.
Olimar names the Onions such because they look like the onions in his favorite soup.
Once you find a type of Pikmin, their Onion comes with them. If you find blue Pikmin, for example, you automatically have the blue Onion.
Just keep defeating enemies and bringing them and pellets into the Onions, and you will eventually get 900 Pikmin. There is no practical reason for having this many Pikmin, though.
Because the most fearsome creatures on the planet come out to hunt during the night, Olimar and the Pikmin have to go into low orbit above the planet after the sun sets. Any Pikmin not in the Onions will be eaten by predators.
No, they did not exist at that time. Only red, blue, and yellow Pikmin are available in the first Pikmin game. There are only white and purple Pikmin in Pikmin 2. They have no Onions, and can only be found by tossing any Pikmin into purple or white Candypop Buds. These flowers can only be found in underground caverns, which were not present in the original Pikmin. The flowers disappear after a certain amount of uses, and thus only certain amounts of purple and white Pikmin can be attained at one time.
5, red, yellow, blue, purple and white leaf bulborbs may be included although they can't be kept in onions and are only found in dungeons
More purple and white Pikmin can always be gotten by throwing Pikmin into Ivory and Violet Candypop Buds, respectively. For the colors of Pikmin that have Onions, a single seed of that color will pop out at the beginning of the next day after what is called a "Pikmin Extinction".
All five original pikmin have been confirmed by Mr. Miyamoto. In addition, as you have probably already seen,, are the rock pikmin. These, despite what people have been saying, are not like purple pikmin at all. Really the only similarity is they give a little umph when landing on an enemy, and even then, they attack by repeatedly throwing themselves at the enemy instead of latching on, and don't have the possibility of making the enemy dizzy, like the purple. A flying pink pikmin with blue eyes can be seen in the trailer for a brief moment at the beginning and end. It doesn't really give you a clear view, though. When confronted about it, Nintendo said they couldn't confirm nor deny the pink pikmin. They did, however, confirm that new unannounced pikmin would be seen later in the game.
The second to last bonus in Pikmin 2 is the complete end when you get the final treasures in the Titan Dweevil treasure group. It depicts Captain Olimar and The President flying off The Planet of the Pikmin with the three Onions following as they ascend to low orbit. When the ship gets far enough from the planet, the Onions stop following Olimar and the President. After, the Pikmin of the original colours (red, blue & yellow) start to glow from the top of their leaves/buds/flowers. The ship flies off, and the planet is shown with "Pikmin 2" on it for about three seconds. After that fades away, the screen says "The End" with the fading planet in the background.
His name is Steve
The red pikmin don't have names their just red pikmin, but if you like there is a famous let's player who calls it Steve so by all means do so yourself.
Olimar named the Pikmin such because of their similarity to the red, blue, and yellow pikpik carrots in his favorite soup.
Olimar's ship in the original Pikmin is called the S.S. Dolphin (after the code name of the GameCube).