Jack-O' Valentine is a key character in the Guilty Gear series, a fighting game franchise known for its unique characters and intricate storylines. She is an artificial life form created by the scientist Asuka R. Kreutz, also known as 'That Man.' Jack-O' plays a crucial role in the series as she is one of the three 'Valentine' models and serves as a vessel for the resurrection of Justice, a powerful antagonist. Her playful yet unpredictable personality adds depth to her character, making her both endearing and formidable in the series.