Who is Iago the Panther from The Girl Who Circumnavigated Fairyland in a Ship of Her Own Making and what is their importance?

Iago is the Marquess's pet, a powerful black panther with the ability to fly. He is, however, as he himself says, still a cat, and able to behave in some very contrary ways - contrary, that is, to the will of the Marquess. Early in the narrative, he offers to free Ell, while later in the narrative, he tips off September to the Marquess's true identity. Ultimately, though, his loyalty to his beloved Queen Mallow remains, even if she has devolved into the evil Marquess, and the narrative suggests he protects her as the Lonely Gaol collapses around her.