When I cast another player's prepped spell, who is 'you' and where does the spell go?

Question

Accepted Answer

You are 'you' on that card: you make its decisions, take its benefits, pay its costs and deal its damage, and 'your hand' means your hand. The card itself lands on its owner's discard pile, meaning the mage whose breach held it. The breach's own bonuses, such as +1 damage from an open III or IV, still apply to the caster. Spells that count open breaches count the caster's.

Aeon's End

When I cast another player's prepped spell, who is 'you' and where does the spell go?