Ally

Ally cards give the Hero players special bonuses to use during the next battle. Player redeem these allies by winning certain battles or by achieving a side goal during a particular battle. When the Hero players get the opportunity to "call upon an ally", this means they looks at all available Ally cards and choose on of them. They gain all benefits listed on that card during their next battle.

Do note that some Ally cards such as Raphael will use the Action Dice of that character. This means that you can't choose that ally. Even though it is not described in the rules, the designers probably meant that you can choose any ally that is not already represented as a hero.