Emma - who plays super swot Hermione Granger in the film series - wears a red frilled dress by the British designer in the latest film in the series, 'Harry Potter and the Deathly Hallows' and Jenny said she is delighted the character has finally developed her own sense of style.
Jenny told Vogue.co.uk: "Designing for Emma Watson as Hermione was so much fun. She has now developed such a sophisticated style, so I had to combine the youthfulness of Hermione, with Emma's iconic fashion status."
She explained part of her success is down to always considering her audience carefully.
Speaking of Emma's red dress, she added: "I designed something that reflected youthful frivolity as I knew young girls all over the world would want to copy this look.
"The vibrant red really pops off the screen and looks magical amongst all that dark sorcery and wizardry."