Warning! Spoilers for The Haunting of Bly Manor below.
The first ghost to haunt Dani, also known as the mirror ghost, in The Haunting of Bly Manor was that of her deceased fiancé, Edmund. But unlike the spirits that reside in Bly Manor, he wasn't a literal ghost. Instead, Edmund was a manifestation of the overwhelming guilt that consumed Dani.
Throughout the first few episodes of Bly Manor, Dani was plagued with visions of a ghost with orb-like eyes. An early episode dove into Dani's backstory and revealed she was previously engaged to her childhood sweetheart, Edmund. But all the while, Dani was slowly realizing she was gay; she was too scared to come out to her fiancé. She finally told him in a heated discussion in their car. He stormed out without paying attention to his surroundings, and was fatally struck by a truck. The reflection of the car's headlights in his glasses gave his eyes the appearance of otherworldly orbs when Dani saw him afterward.
Bly Manor is just as much about metaphorical ghosts as it is physical ones. Visions of her fiancé haunt Dani in every sense of the word. The Hill House follow-up series is all about emotional trauma; this particular ghost is the clearest example of that. It's not until Dani figures out how to move on that she can fully heal, which is the real reason why Edmund's ghost finally leaves her. Dani also gets a little help from her new love interest, Jamie, the gardener at Bly Manor.
Jamie Made the Mirror Ghost Disappear
Edmund's death, and the circumstances that indirectly caused it, have caused Dani a great deal of stress and guilt. It was Dani's admission of her sexuality just a short amount of time before their impending wedding that caused Edmund to storm out into oncoming traffic. Because of that, she blames himself for his death. Dani carries Edmund's glasses, cracked from the accident, around as a reminder. She seems that final image of him, with the truck's headlights reflected in his glasses everywhere — mainly mirrors.
That mirror ghost sticks with Dani because she can't let go of her guilt — guilt that she never told Edmund about her sexuality before, and guilt that she's the reason he walked in front of a car. The guilt completely takes over her life, which is why she runs away to England for a fresh start. It's not until she meets Jamie at the ghost-filled Bly Manor that she begins to heal.
The show offers hints that Dani was always expected to marry a man, so there's a good chance that her family would not have accepted her sexuality or approved of her having a romantic relationship with another woman. But once Dani met Jamie at the beginning of The Haunting of Bly Manor, she finally feels safe, accepted and loved. She always loved Edmund, but just as a friend — Jamie offered her something different. Once she feels secure in herself and her relationship with Jamie, Dani throws Edmund's old glasses into a bonfire. As the mirror ghost is the physical manifestation of her guilt and sadness, that small act of ridding herself of his glasses is an important turning point in Dani's journey to self-acceptance and forgiveness.