Cu is a big furry sheepdog, with a puff of fur covering both his eyes.
Cu first appears as a pup with 4 year old Ben as Ben's parents, Conor and Bronagh put him to bed in the prologue.
6 years later, Cu remains best friends with Ben but he also loves Saoirse and shows concern for her such as going after her when she is drawn to the presence of seals, much to the chagrin of Ben who fears water. Ben confides in Cu whenever he feels ignored in favor of Saoirse or is in trouble for giving her a hard time.
When Ben and Saoirse's grandmother takes them away to live in the City, Cu goes after them and finds the kids trying to find their way home through the Irish countryside. Believing Cu can lead them home, Ben and Saoirse attempt to follow him back the way he came but they are forced to take shelter in a Holy Well as it starts to rain and Saoirse falls ill. Saoirse then jumps down the well and Cu goes after her, pulling Ben in with him until his leash snaps.
Cu somehow manages to find Saoirse who has been captured by Macha the Owl Witch and comforts Saoirse as she slowly turns to stone. When Ben finds and confronts Macha, he discovers Cu and Saoirse in Macha's attic. Cu blocks Macha from entering the attic as Saoirse plays the selkie song on the shell flute at Ben's urging. This releases Saoirse and with help from Macha's son Mac Lir's dogs, Cu carries Ben and an increasingly-ill Saoirse back to the Lighthouse.