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Mercy is Wadsworth's pet bat. He found the morning after Ouster Eve as he was walking with Dr. Angstrom. Seeing that she was shivering and all alone, he took her to the antique shop and put her in a bird cage, "just until you're feeling better". He also hoped she wouldn't get big, like the giant bats the Bottlefly Boys were able to dive away during the Ouster Eve celebration.
Mercy has become very loyal and devoted to Wadsworth. After he was attacked, Mercy refused to leave his side and accompanied him on the trip to Amaranth General Hospital. She also distracted the spirit of William Harker when it had kidnapped Wadsworth and was holding him captive in his house, giving Wadsworth time to escape. The spirit struck her, injuring her wing.