Twilight to Moonlight
The secret chapter "Prank" is essentially an early version of what would become the chapter "Henshitsu (変嫉)" in the parallel universe sequel Moonlight Syndrome.
After Goichi Suda's work on Twilight Syndrome he was given the resources to create a game now entirely written and directed by himself. He pivoted towards telling a story concerned with psychological horror, involving human characters as the main motivators of the plot, and have the horror come from the horrible things that people can commit instead of a fear of the supernatural (a fear that Suda, in fact, had.)
With strong inspiration from the works of David Lynch infused with his fascination with the "Heisei malaise" and lost years that affected the underground sub-culture in Tokyo and the era's youth, Suda created a story concerned with the lives of high schoolers whose lives are irreperably changed after an event fully uncovers a darkness that begins to develop in Hina-shiro, concerning one Mika Kishii...
Unfortunately, development of the game was troubled by the aftermath of the 1997 Kobe child murders which lead to parts of the game being self-censored by Human Entertainment to avoid backlash from the press and Japanese government. A large majority of the game's planned content was also left in the conceptual stage as there wouldn't have been enough time nor budget to fully realise it, as Human Entertainment was teetering on the verge of bankruptcy until it would fold in late 1999.
A version of the original planned story was used as the basis for the novel adaptation of the game.
Two virgin steeds
Zoroastrianism was a major inspiration for Goichi Suda when creating what would eventually become Moonlight Syndrome. The early workings of that can be found in this chapter.
The "two virgin steeds" could be referring to Tishtrya and Tiri, two gods that are usually depicted in the form of horses, but their relevance to the story is unknown.
In the Moonlight Syndrome Deep Guide, Suda elaborates slightly on the meaning of what Ryo says in this chapter.
""When the moon's sorrow overflows~" was something I wrote at the start with the image of Moonlight Syndrome in mind. It comes from the moonlight in the title, and hints at the events of the epilogue."
"To someone who forgives all" is a feeling of self-admonition that Mika Kishii has towards herself for her actions (pranks and making up stories) in Twilight Syndrome. It also signifies the question of whether or not Ryo will forgive Mika as a person.
Can Ryo stand to protect a woman Sumio had relations with, despite feeling something like a mix of love and hate for him? It's a representation of this jumbled relationship between Mika and Ryo."
(Translation by FFTranslations)