Stones

Stones are used to increase the base modifiers of the characters and to increase their "Star Level". The stones look like portraits of the characters. They must be used in groups of three to upgrade your character and generally come in packs of five. Different amounts of stones are needed to unlock and upgrade different characters for each star level.

to Unlock a character: to Upgrade a character:
 * One Star: 10 stones
 * Two Star: 40 stones
 * Three Star: 100 Stones
 * Four Star: *not unlockable*
 * Five Star: *not unlockable*
 * one > two stars: 30 stones
 * two > three stars: 60 stones
 * three > four stars: 120 stones
 * four > five stars: 180 stones

Character Stones can be found in many places. Some shops have a set list of character stones they sell where as some shops sell completely random ones.
 * Daily bonuses and giveaways
 * Elite battles
 * Shops
 * Adventure Extra area
 * Vegeta Extra area

Any one Elite battle can be done three times per day and has a small chance to drop a stone of the featured character. (I normally get 1/3 fights drop a stone. Very rarely do i get 2/3 and I have yet to see 3/3)

Also "Beast Soul Stones", which look like a Red Ruby, can be found and are used to "Sacrifice" to your characters.