"Mad" Harriet Marchweather is a boxer and a thief, as well as the FLPC in Batty's timeline. She's its long-suffering best friend, and has supported it since even before it wiped its memories of its childhood. It was careful to preserve its memories of her even if that meant letting a few unpleasant memories stay for context to their relationship. Harriet views herself as a maternal friend towards Batty, and in the past frequently attempted to find Batty a man to love. Due to Batty's nature and lack of understand of social niceties, this failed for a long while. While Batty did end up marrying a man Harriet introduced it to, Harriet did not at all intend for them to be introduced as romantic partners for one another and is rather baffled by that particular turn of events.

Unlike the canon FLPC, Harriet was born in the Neath.