“The misanthrope lives!” – ZUN
In Touhou’s past, Reimu clashed regularly with a vengeful spirit wielding powerful sorcery and an intimate knowledge of the Hakurei Shrine’s secrets. This spirit was Mima and in the transition between the PC-98 and Windows canons, she vanished without a trace.
In Highly Responsive to Prayers, the first Touhou game, Reimu initially encounters Mima as she breaks free from a smaller shrine connected to the Hakurei Shrine. Though they don’t exchange any words at this stage, Mima’s appearance may provide some insight, specifically the bloody knife she holds.
This knife isn’t visible in her future portrayals and it’s never discussed in any canonical materials, but it may hint at her reasons for turning into a vengeful spirit. Perhaps she was murdered and unable to find peace or, given how she possesses the knife, maybe she’s the spirit of a murderer.