Karen McCombie: Certain Dark Things [2004] paperback

Stella, along with her mun, dad and mildly mad twin brothers, has just moved from buzzy Kentish Town to the sleeeeepy seaside town of Portbay. Not only has she had to leave her best mate Frankie behind but she's also kissed goodbye to Seb - only the guy she's liked for ever...Still, Portbau's got its own charms; they might just take a little discovering. Luckily Stella seems to have acquires a mysterious purry, furry companion to help her find her feet...