If you havent checked yet, there are also leaked backstorys..
you can find them here:
http://www.playdota.com/forums/540621/dota-2-hero-stories-pudge-hooking-tutorial/This one is for Storm Spirit, explaining why is he so whimsical:
Storm Spirit is literally a force of nature--the wild power of wind and weather, bottled in human form. And a boisterous, jovial, irrepressible form it is! As jolly as a favorite uncle, he injects every scene with crackling energy. But it was not always thus, and there was tragedy in his creation. Generations ago, in the plains beyond the Wailing Mountains, a good people lay starving in drought and famine. Qazmyr Thurifex, a simple elementalist, used a forbidden spell to summon the spirit of the storm, asking for rain. Enraged at this mortal’s presumption, the Storm Celestial lay waste to the land, scouring it bare with winds and flood. Qazmyr was no match for the Celestial--at least until he cast a suicidal spell that forged their fates into one: he captured the Celestial in the cage of his own body. Trapped together, Qazmyr's boundless good humor fused with the Storm Spirit's crazed energy, creating a jovial Celestial who walks the world in physical form.