im sorry for the confusion. its the king from a different country/kingdom during the war. but just a bit of spoiler that i think was hinted already. his brother, the current emperor, had something to do with eileen's death too.
note: next infos will be before the beginning of the story where cesare came back as a hero.
basically on the ORIGINAL timeline, when cesare was sent off to a war with a different kingdom, he was supposed to take down the whole kingdom but spared the enemy king and negotiated instead. this gave the enemy king a chance to pull the nobles from the empire who were on his side to take down cesare since he has more influence and power than the emperor, and everyone (including his brother) was threatened by him. he was like a ticking time bomb. this led to eileen's death. so when cesare came back from war, she was already beheaded and people took her body parts as a "lucky charm" since shes a noble. they even displayed her head in one pub (that he blew up in both timelines).
the CURRENT timeline is where he beheaded the enemy king without negotiation, avoiding him from orchestrating eileen's death. and since the nobles who were supposed to team up with the enemy king suddenly lost their support, they are being weeded out by cesare one by one hahahaha. he still blew up the pub despite eileen being alive, and married eileen to protect her from the nobles. his subordinates actually found it weird that he never even tried to listen to the enemy king but since he already knew what will happen, he didnt care and just killed him.
also eileen was the target because they found out from a "source" that cesare deeply cares for eileen. he was never one to possess or covet anything (like the throne) so when they found out that he doted on her and even sent her to the academy, they immediately placed her on the guillotine. it was also told that he "raised" her so you may read the word "child" from cesare a lot :). it really sounded weird at first but you'll find out more once you read further!
also, for the part where cesare caused eileen's deaths. it has something to do with the myth mentioned in the manhwa already! about the 7 days 7 years something hahahahahhaha. its too painful and too much of a spoiler so let me know if youre willing to know more about the connection of 7 myth and eileen's death before i proceed :3
the 7 years, 7 days, and 7 trials myth was explained in ch 32 if you need to read it again! ill be explaining further the connection of the myth to the original timeline
when cesare came from war and found out about eileen's death, his world collapsed. she was like the light in his life that made him feel human. he basically went berserk and murdered all the people involved in her murder.
i believe it was mentioned that cesare's mother is very superstitious that him and his brother became skeptical. they were forced tovdrink questionable liquids for her rituals. however, due to cesare's desparation ro nring back eileen, he started getting into rituals until he found out about the 7 years, 7 days, 7 trials of the founding king.
im sorry it wasnt clear or maybe i couldnt remember but as far as i know, cesare went on a killing spree for 7 days with his subordinates and offered the dead bodies on the altar until the god brings back eileen. on the first day it was a couple of dead bodies, until body counts grew large and when they were killing thousands on the 7th day, the god answered cesare and told him to go through 7 trials.
the 7 trials was to kill eileen each trial....
(tw suicide)
the first trial was when he met eileen when she was around 11 crying. cesare was not aware of the specifics of the trial so instead of killing eileen, he shot himself in front of her. however despite the multiple suicides, nothing changed in the scenario. he realized that he had to kill eileen in order to move to the next trial.
at first it was easy. he could handle killing her for the sake of saving the real one. but the more he goes through the time they spent and kill her each time, the more it damaged his head. and just a quick note, in every trial, eileen gradually grows older. so on the 7th trial, eileen was already a fine woman living in the brick house. cesare was so drawn to the comfort of eileen living with him there that he stayed. personally, i think this is the part where he started seeing her as a woman. they were very domestic and even did it numerous times. he craved for eileen so much he didnt care if she was real. but he realized she was still a dream and he needed the real one. the eileen in trial is repetitive compared to the real one who showed real emotions. so on the night when they were in bed, he killed her and she thanked him :)
after the trial, he came back to the new timeline where he beheaded the king!!! so yeah hes very unstable after going through the 7 trials (and several suicides) !!! not to mention the mass killing he did !!!! (人 •ᴗ•)

i read the novel when this just started and found some comments saying that they couldnt understand whats happening. i think its because there are a lot of jumps from the novel.
a bit of spoiler + addtl info from the novel to explain whats happening right now !!
so to summarize, while cesare was at the war, he actually came back a few years back and changed the death of the king to save eileen. instead of bargaining with the king who would ultimately get revenge and orchestrate a revenge with the nobles by killing eileen, he sliced his head off immediately without any negotiation to finish the BEGINNING of the reason why she died. to keep her safe, he married her and made her more influential so that she wont be easily disrespected. cesare is actually very unstable because he saw her death and CAUSED her deaths. so right now, hes weeding out all the nobles who led to her death.
aside from that, he's actually more physical with her in the novel. they shouldve done it a lot of times already. he was supposed to sloppily make out with her during the bridal kiss to the point that everyone had to look away and they did it at the brick house too. lots of intimacy because whenever eileen starts to feel upset, he starts doing it with her. its actually a very important detail in the novel that shouldve been added but they had to make this censored sigh.
i hope you guys wont drop this because its a very compelling story. you may find eileen annoying (i did ) for the upcoming chapters but if you actually analyze her character, she has a valid reason for being "mild" "boring" or even "weak".