She is a playful and innocent kitten, but she is more mature than others kittens. As we can see in The Moments of happiness, Jemima/Sillabub is the only one apart Old Deuteronomy who understands what happiness really is. I think she's got a sort of connection with the moon and the dawn. Like Victoria, she understands who Grizabella really is : a Jellicle, even if she left the tribe a long time ago. That's why you can see her trying to touch Grizabella each time she appears. When she sings her little part in Grizabella, her look and her voice are full of friendship for the old cat. Her solo in Memory helps Grizabella to be accepted back in the tribe.
On that night, the Jellicle cats recieve a great love and forgiveness lesson from Jemima and Victoria.