After breakfast, Ferdinand noted that Arabella's condition was truly stable. Thus, he informed her that the one who likely added poison to her wine was not the butler who served her the wine.
Instead, it was one of Lucille's maids called Risa.
They learned of the said maid's identity through a thorough investigation of all the maids and butlers who served as waiters and waitresses during the banquet. Even the kitchen staff and guards assigned to the venue were investigated.
Risa was only identified after all the palace maids and kitchen staff were gathered and they noticed two people were missing among those who helped them with the food and drinks that day.
Through the descriptions of those who saw her, Alwin made a portrait and it matched Risa's appearance. They confirmed it was Risa who was missing when she was brought to them.