HC: Reimu is actually NOT a legitimate descendent from the Hakurei bloodline. She was adopted by the previous Hakurei miko as she refused to have a child directly. Yukari knows about this as she was tasked to raise Reimu alongside Shingyoku after the sudden demise of the previous Hakurei miko.
This explains why she has not fully mastered her abilities as the Hakurei shrine maiden and the reluctance of the Hakurei god to interact with her, as well as her noticeably different attitude compared with previous Hakurei mikos. Reimu doesn't know this however, and Yukari is planning to keep it a secret for as long as she can so to not pressure Reimu any further.














