Alya Césaire is Marinette's best friend. She constantly helps Marinette with her problems, most of which pertain to Marinette trying to speak to Adrien; she is often seen helping or encouraging her. She aspires to be a journalist and is in charge of the school blog. She is a big fan of superheroes, in particular, Ladybug; she even operates a fansite known as the Ladyblog. Alya was initially unaware of Marinette's double life like everyone else before the fourth season when the latter confines her identity as Ladybug to her.[15] Since then, Alya continues to support Marinette in her new role as Guardian of the Miraculous.

She usually uses the Fox Miraculous to transform into "Rena Rouge" ("Red Fox"), armed with a flute that doubles as a staff which allows her to create illusions through her "Mirage" ability.[16] Although her identity is eventually revealed, Alya is called upon for support as Rena Rouge once before being eventually allowed to keep her Miraculous, assuming a blue-colored variant of her hero form called "Rena Furtive" ("Secret Fox") to protect her identity until she returns the Miraculous to Marinette once her identity is exposed again. In "Re-creation" after Marinette recovers the stolen Miraculous and reforges them following Monarch's defeat, Alya is given back the Fox Miraculous permanently and reunites with Trixx.