Well, a few things, and this is just how they start off in the game. Anno has, I think, the right idea about where they go, but if you want to know how to start...
First off, OCs and Canon characters can be from different worlds, but only OCs can have their home world be Earth for obvious reasons.
One, if they're from another world, the only way to get to Earth, at least one that's revealed (because one of the NPCs has hinted through the fact that he has SPARTAN armor and has talked with some certain characters that there may be another way), is to be drafted/contracted into the war by either Cosmos, the Goddess fighting for Hope/Good, or Chaos, the God fighting for Despair/Evil. One thing to note is that they don't do it in a Deal with the Devil sort of deal. Cosmos goes and asks if the people wish to fight in the war while Chaos cherry-picks which ones he'd want in the war, not based on their powers or skills, but how they'd enjoy causing Chaos.
Characters born on Earth are different. They're muggles. They're brought into the war in another way. Maybe two. The first way is to pick up a Power Key... I think that's what it's called. Basically, it's an action figure of a character which contains its power. For example, my character, Perry, got a Power Key in the form of a Power Ranger (okay, Super Sentai character, but for all extents and purposes...) and activated the doll, gaining the ability to morph into a Power Ranger. From there, they're recruited into the battle by either Cosmos or Chaos. Mostly depending on who made the Key in the first place (people, when declining that offer I mentioned above, have the option to at the very least give a copy of their powers to be used in battle).
Or, and this is exclusive to Cosmos characters, they could stumble upon a laboratory stationed right near a giant spiral drill tower thing. The organization in this lab is called AEGIS, and they so far have at least one or two characters stationed there. Basically they're the guys you go to for stuff like SPARTAN armor or cool guns from other worlds. Once you joined a faction... yeah, that's pretty much getting into the game.
Alright, so, canon characters are not actually from Earth, but some form of alternate dimension and OCs ARE on Earth but get their powers from an action figure? Seems simple.
Gonna get started on my applications, then.
Yeah. An alternate world that's based off their stories. It works sort of like Kingdom Hearts in that regard.
edited 16th Aug '13 8:21:55 PM by Psyga315
-frantically trying to get Gaige some character interaction-
It'll also probably help get me back into the swing of things here.
edited 1st Sep '13 7:29:30 PM by SpartyMcFly
"Seven is here too, dressed like the concept of choosing clothes that look nice together was an arcane secret far beyond their grasp."

I'm definitely still here.
"Oh, dear. The toad, the monkey, and the dog have all screwed up."