Gary was a powerful Wizard in Faerun. But, when Mystra died and the Weave broke down, he realized his magic no longer worked the way it should. In the hopes of learning how to use magic again, gary made a deal with a powerful archfey. She instructs him in the new magic, but she's also invested in him a piece of her own power. (Wizard, multiclassing to warlock)
Beth grew up on the mean streets of Fallcrest. She learned how to pick pockets with the best of them, and she was quite adept at avoiding a fight or, when she couldn't, making sure it ended quickly and in her favor. When the Iron Circle started their campaign of destruction in the south, though, Beth realized she was needed for something more. She signed up for the Fallcrest militia, and there she was trained as a proper soldier. (rogue, multiclassed to Fighter)
Jill was a soldier in the Nerathi military. During a long campaign against Karkoth, Jill's group was overrun by undead, and she was the only survivor. She has no idea how she survived, but she was saved by a tribe of what those in the capital called barbarians. One day, Karkoth troops under the command of a lich were spotted within a few miles of the tribe's home. Jill decided to repay the tribe for their kindness to her and joined them on an excursion to rout the invaders from their territory. After the battle was joined, jill knew how she had survived: the tribe's prowess in combat was matched only by their ferocity, as their rage drove them to shrug off countless blows that would have felled lesser men and fueled their arms to create vicious swings of their axes. Jill accounted well for herself in the battle, and the tribe offered her a place among them. She accepted, and she began training in this new style of combat, using her greatsword in place of the tribe's favored weapon. (Fighter multiclassed to Barbarian)
Mortimer's mother was a druid, and she brought him up in the same way, teaching him the ways of Nature, the ways of the animals, and now to thrive on what the world could give to you rather than what you could take form it. In his journeys, Mort heard the name Obad-hai. People who worshiped Obad-hai also loved Nature as he did, and they said that Obad-hai was its creator and protector. Spending some time with a cleric of the god (who unfortunately later tried to stand alone against a white dragon and was frozen and eaten for his insolence), Mort himself became a believer, and he pledge his life in service to Obad-hai. (Druid multiclassed to cleric)
I really don't like stuff like Fighter/Sorcerer/Cleric/Druid/Monk/Barbarian. A character that ends up doing that is generally either min-maxing (which isn't a huge deal, but I don't care for it to this extent), or has MPD or something. Most characters aren't going to stray too far from their original class, as that should generally be the one most suited for them anyway. Barring completely life-changing events, that is.