Pretty it's one of those "take over someone's body, make their life much better because regrets." I don't think the system plays a major roll. Not 100% sure it's ABO, but I'm quite positive it is. Rather than being born as an A/B/O, it was more like the people absorb the strength of those they defeated and then the end result would classify them. Host was an orphan, picked up to be a sacrifice for a young master. The sacrifice would do lots of the hard work, earn strength then purposefully lose. The young master was inconsiderate and selfish (I don't think he was ML). When they were young he promised that they would both be alphas together,he forgot about it, told host to lose. When host turned into an omega, YM would still take his anger out on host, but unlike before, host's body is weak. He even took host with him when he was deployed, which was bad because most soldiers were alphas. In the second timeline MC gets away from YM, YM isn't so ignorant and does well without the need of a sacrifice. I think YM dreamt of the first time line and hates the awful things he saw himself do to the host. There's too many transmigration BL, I've lost track even after I stopped looking at forum spoiler threads of yet translated novels. (´-﹏-`；)
Oh shoot, it really was that? I crossed it off immediately because I thought the system didn't have a personality in it. Although it does explain why I feel like the young master was given a second chance. (Which, would mean that MC didn't end up with their multi dimensional big tigh ML.) Thank you so much! Edit: Yep, it’s the right novel