Some SPOILERS . . . . . . . . . Sanjou is not a love rival. There was nothing of that sort happened between Kaburagi and him. The “dark past” was nothing bad, just that Kaburagi is not very proud of how he became friend with Sanjou to use him and he absolutely wants to leave his past behind with everyone that was part of it (like how he ran away from his own sister). Onoe and Kaburagi remain a very strong pair and nothing seems to be able to faze them. There are a lot of meaningful communication, they are very straight forward to each other and in the end, Kaburagi came to peace with Sanjou and life is good again.
Raws: https://ba-raw.livejournal.com/tag/ameiro%20paradox (from chapter 13). Please consider donating to help them!
Some SPOILERS
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Sanjou is not a love rival. There was nothing of that sort happened between Kaburagi and him. The “dark past” was nothing bad, just that Kaburagi is not very proud of how he became friend with Sanjou to use him and he absolutely wants to leave his past behind with everyone that was part of it (like how he ran away from his own sister). Onoe and Kaburagi remain a very strong pair and nothing seems to be able to faze them. There are a lot of meaningful communication, they are very straight forward to each other and in the end, Kaburagi came to peace with Sanjou and life is good again.