Denis was born in 1946, to Proinsias Cassidy.

Cassidy couldn't stand being immortal and seeing his love ones grow old without him, so he abandoned Denis at an unknown point.[1] Many years later, Denis begrudgingly let Cassidy and his friends Tulip O'Hare and Jesse Custer stay at his apartment.[2] Denis was dying, and wanted his father to turn him into a vampire so he could live forever. Though Cassidy initially denied his request, he eventually did the deed and bit Denis.[3]





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