Who is Thessala from Arthurian Romances and what is their importance?

Thessala is Fenice's nurse. She worries about Fenice's ill-health before marrying Alis, but when Fenice denies being sick, she recognizes signs of love and offers to give Alis a potion that will make him think he has possessed Fenice when he has not. Thessala's potion works.

When Fenice and Cliges reveal their love for one another and decide to elope, Thessala assists Fenice in faking her own death, and then she lives with her mistress in a secret house. When Cliges and Fenice are discovered together, Thessala helps them escape.