Cait Sith is a large humanoid cat with mysterious powers that resides in Neo Venezia. He has been the king of the cat kingdom in Aqua for at least 100 years. Throughout the series, the people of Neo Venezia share many folk stories on how the cats of city contribute to its mystique. Cait Sith is said to be the leader of these cats, creating fantasies for those individuals deemed deserving and suitable to bear witness to them.
In the manga, it is hinted various times and ultimately revealed that, as he secretly guides Akari (her alone) to the seven mysteries of Neo Venezia, he is in fact Akari's pen pal and the final mystery.
In the anime, Alicia and Akari suggest that he is the spiritual guardian of Aqua. Junichi Sato, the director of the anime, claimed that his interpretation is "a little different from the original manga's interpretation".
In Aria the Avvenire, Akari tells Ai how she gave Cait Sith her last goodbye, since only children can see Cait Sith and after becoming a prima, she would be considered an adult. Cait Sith gave Akari a parting gift - pendant with blue stone (probably an Aquamarine).
"Sooner or later, you have to give goodbye to everyone you meet. But even though I sad goodbye to Cait Sith that day, I think Ai has helped me see him again. Maybe love is what calls forth miracles."