"I lost Theo! I lost my best friend! We were playing over by the river, and Theo said he wanted to go look at the big green thing. I said we shouldn't, but we snuck over there, and the suddenly this bug came out! We ran away but Theo fell down and the bug grabbed him and took him away!"— Celia(src)
Celia was an inhabitant of Tristram.
Celia held great attachment to a teddy bear named Theodore ("Theo" for short). During the Darkening of Tristram, she and Theo were playing by the River Talsande. Theo 'told' her that he wanted to look at the "big green thing." Celia didn't, but they went over anyway. Suddenly, a large insect came out. Celia ran away, but Theo fell (a.k.a. was dropped), and the insect took him into the depths of the Festering Nest.
Celia is exclusive to Diablo: Hellfire. She initiates the Little Girl quest. Note that she can only be seen if the player uses the Command.txt exploit. She appears on Adria's island, near the bridge, but not until the player has entered the Festering Nest. She is also one of the few NPCs who changes appearance in the game. After the player retrieves Theo for her she can be seen holding him in her arms. Note that if the Hork Demon will drop a random amulet instead of Theo if slain. If this the case, 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.