Spoiler
Ido was his husband in TL1 he got to Ido’s bad side because of his family’s scheme and thus Ido abused him physically and mentally. He r@ped him and treated him like a wh@re. The brother also did r@ped MC and days after that Ido also did it and forcedly imprint him. The ML was able to share MC’s memories and feelings and that’s how he knew that Senjin has been sincere this whole time. He tried to make it up to him but Senjin was already too depressed, added to the fact that he got pregnant and doesn’t know who the father is, he wen to Ido’s study and gunned himself and Ido witnessed it.
He did not, he was planning of exposing his brothers crime, so he told his brother that he has a gift at home and told him to get it. Ido thought that Sejin won’t leave his room and him and his brother wouldn’t have any chances of meeting but Sejin’s heat came and he has to go out of his room to the bathroom and there Ido’s brother found him and did the dead! He told Sejin that he was the gift that Ido gave, so Sejin thought that Ido abandoned him and after the SA Sejin fled but it was only 2 days that Ido found him and forcedly imprinted him. Ido gained him memories and feelings and that’s when he knew that Sejin’s feelings were true. He took Sejin home again and didn’t appear in front of him but he stopped harassing Sejin. Sejin was severely depressed that he no longer eats, they have to put him on IV so that he could continue living. Then a month later he found out he was pregnant but he doesn’t know who’s child it was and after that he forced himself to get up and walk to Ido’s study and there he gunned himself. He was taken to the hospital but didn’t make it. Ido soon followed him to his death afterwards.
I think they used to be lovers,but he got raped by Ido’s brother, got traumatized and lost his memory