She. The creature is female, which seems more important than the fact that Shen Qingqiu betrayed Elias, or at least that melancholy that comes from Elias implies this quite heavily. Still, Elias isn't acting like he has something to be ashamed of, if anything, he seems concerned about the creature he has contained.

"If she's safe, and we are safe from her, then I see no need to continue the conversation here."

She eyes Elias.

"Should she become a threat to the town, I will not hesitate to kill her."

This is the truth just as much as it is said to see how he reacts.