What cocktail ingredient is primarily used to make a Kir Royal?

The primary ingredient in a Kir Royal is Champagne. This cocktail is traditionally made by combining a measure of Crème de Cassis (a sweet blackcurrant liqueur) with Champagne. The effervescence of Champagne not only elevates the flavors of the liqueur but also adds a celebratory, festive quality to the drink, making it popular for special occasions. It's important to note that while Crème de Cassis provides the flavor and sweetness, the Champagne contributes a refreshing, bubbly element that defines the Kir Royal.

Using other ingredients like brandy, soda, or bitters would not create the same drink and would alter its essential character. While brandy is a key component in some cocktails, it does not feature in a Kir Royal. Soda adds fizz but lacks the complexity and tradition associated with Champagne, and bitters offer complexity but do not complement the sweet notes of the Crème de Cassis in the context of this cocktail. Thus, Champagne is crucial to achieving the desirable balance and experience of a Kir Royal.