
[Major spoiler for this prequel story]
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The ending of the story from the prequel is tragedy, that's why the main story is the reincarnation story (after 300 years). Sirius was only shown through flashbacks so far.. His reincarnation is still unknown so far, since there's no one who have memories of their previous life except for the MC saintess (but my guess would be the captain of the 1st order).
There's a hint of mystery in the main story of what will happen if other knights from the past (white & blue) also have memories of previous life or reincarnated like the MC saintess and what will be the outcome of the MC's life since there's someone one who wants to revenge against her.
This is the prequel story of The Reincarnated Great Saint Hides Herself as a Saint. The beginning story of Seraphina from Sirius' the white knight's perspective? (not sure who's perspective from).
If you have read the other work, you'll know this story's direction (ending).