One of the aspects of writing I enjoy the most is the discovery of new characters, ones I hadn't planned on or ones that decide to play a much larger part in a story than I had anticipated. I write stories "into the dark." (That's not my term. It belongs to Dean Wesley Smith.) I start with a situation and characters and some vague idea where the tale might end. My best ideas come while I'm putting pen to paper. The vague endpoint becomes a moving target. The story meanders in unexpected directions and usually becomes much longer than anticipated.
Other characters emerge out of necessity to play a bit part but then they won't leave. Liesael came about because I needed some members of a family with a sick patient that Merliss and the cunning men were visiting. Liesael revealed a love interest in Fendrel, the cunning man's apprentice, and slipped a love charm into his satchel. She later came to stay with Fendrel and nurse the cunning man when he became ill. Liesael doesn't play the prominent role that Slynid does--more of burr in Merliss's fur--but she grew from a mere decoration to a significant player.
Late in the novel, Merliss escapes through the woods, traveling from one scene to another. I thought to add some interest with a chance encounter with another cat. Tawk is the name of a very timid feline who barely escaped his own death. Tawk gave me a chance to depict events in other villages. Merliss finds him annoying and an object of pity. I thought Tawk's first scene would be his last but like Liesael, he refuses to leave the story and tags along to the very end.
Slynid, Liesael, and Tawk will certainly appear in future Merliss tales. I don't know where they came from but I'm very glad they appeared. I know I couldn't have planned them or their exploits ahead of time. Writing is a lot about trusting that the characters you need will appear when you need them. And you have to let them do what they will, even if it throws a wrench in your original plans. The fun part is getting to know your new characters.