Celia was a little girl in a blue and white dress who gives you a quest to find her lost teddy bear. She gives you a random necklace for doing the quest. Note that she can only be seen if you use the Command.txt exploit. She appears on Adria's island, near the bridge, but not until you have entered the Festering Nest. She is also one of the few NPCs who changes appearance in the game. After you retrieved Theo for her she can be seen holding him in her arms. She was brutally murdered during the attack on Tristam.
Theodore (or sometimes just Theo) is the teddy bear that you are sent out to look for in the Little Girl quest. He can be found on level three of the Festering Nest, where he is dropped by the Hork Demon (the boss of the level). If you do not use the Command.txt exploit neither Celia nor Theodore appears in the game. In that case, slaying the Hork Demon will result in it dropping the random amulet instead of dropping Theodore. Otherwise, an equivalent amulet will be given by Celia upon returning Theo to her. Interestingly, Celia speaks of Theodore as of a living being (which is understandable for a little girl), and until the moment of slaying the Hork Demon, players will not find out that Theo is actually a toy.