Goushou and Ren (their names in the current timeline in prologue and epilogue), and whatever their names are in various reincarnations always have a boy near their side, whose in the initial life is the acolyte with mysterious powers, studying under the Horus priest mentioned above. And every time they reincarnated, they always had a lapis lazuli, tattoos (the seme has eagle tattoo, and the uke has lotus one) and they're "protected" by Goushou's (or Aketo on his initial life as an Egyptian prince)'s eagle.

They took different appearances every time they reincarnated and the seme-uke types differed in every reincarnation.

Here's the lineup of each reincarnation story told in the novels:

1. Prologue: current timeline: Art Dealer & University Student
2. Initial Life: Prince & Priest (Ancient Egypt)
3. Court Minister & Craftsman (Ancient China)
4. German Swordsman & Rome General (Ancient Rome)
5. Thief & Knight (Medieval Europe)
6. Spanish invader's translator & Inca royalty (Medieval South America)
7. Low ranking samurai & Boy in Iron Mask (Edo period Japan)
8. Nobleman & ...(don't know how to translate it) A chivalrous man (Taisho period Japan)
9. Epilogue: Art dealer & University student

1-3 is put in book 1
4-5 in book 2
6-7 in book 3
8-9 in book 4

(by festivewind from Tanbi_ Community)