The Young One is a hearth statue that has been in the family of Closti the Clam for generations. The statue holds at least part of the soul of Tanamil, another member of the Undying who was partly bound when he created the Red River that joins with The One's river at watersmeet.
By carrying the statue of the Young One with her when she and her two brothers sail out to board Kankredin's ship, Tanaqui manages to bring him with them, and it is he who helps them through the Soul Net the first time, and Duck who pipes them through the second time, as he'd been taught to do by Tanamil when the children stayed with him in the winter.
Tanamil is in love with Robin, who had opened up to the joy Tanamil brings everywhere he goes, and had danced with him when they were in his home. However, they quarreled badly near the end of their stay, and he only comes back to rescue her when she has decided to die, and Tanaqui decides she must call Tanamil back to them to try to save Robin.
Tanamil is not as powerful as The One, but he is not as closely bound, either. He can't bear to see his statue, which reminds him of his powerlessness every time he catches sight of it.