>Aglae at the beginning goes to Marinette "Man it's suuuuure sad that Alix can't be around for her own birthday.
>Marinette immediately swaps with Alix so she can have a party.
> Aglae at the party "Huh, Marinette's not here yet is she not coming?" And Alya immediately gets a message from Marinette in the Burrow to make an excuse for her.
>Akuma fight suddenly cuts back to the party to show Aglae has disappeared.
> At the end Lila is back in her lair. "As expected she didn't show, but I need to be absoluetely sure."

Yeah Aglae is Lila.