Plot is that the other tutor was a powerful entity gaining 'revenge' for the first human he met, who was the unknown younger brother of the first head of the house. FL looked really similar to this younger brother, which was why she gained so much favour quickly and was allowed to survive for so long. The demon (I'll call him), was attempting to make her into a replica of the younger brother in a sense. Anyways, the demon's revenge included following the wish of the younger brother to help the first head of house, who was failing financially and taking it out on the younger brother both verbally and physically. So it was a give-and-take, where the demon allowed the house to prosper financially whilst also abusing them and leading them to ruin. Which is why ML was so badly mistreated by the demon. Simply having the blood of the first head of house was enough to warrant complete and utter hatred. This is all paraphrased btw, expect mistakes. Anyways, after the first plan to free the house from the contract fails since the demon breaks the promise he made, there's a second plan in set to just 'kill' the demon, which would just lead him into a long slumber. Things happen, FL grows closer to ML and they decide on things to do once they're free and she stalls the demon whilst ML works on the plan. (Burning the demon's roses, which basically were his essence in a sense. There's more details in the plan I won't touch on). The roses turns out to be a portrait of the younger brother who FL realises looks similar to her, and ML takes her and runs off with her before her distraction ruins her. Since the mansion was burning down. Story ends with them getting on a horse and deciding how to spend ML's birthday, since it was that exact day.

As we've read through the manhwa the fact is Madam Otis, Allen And the MC are real people in the mansion.
Note: it's from the spoilers I read
As for people wondering about Mr. Rogers, he's a demon and those twins? They're just vassals created by the demon as his first victim were the lil kids.
Madam otis knows about this that those are not her kids and she herself is abiding by the rules— shackles made by the demon as his revenge on the Otis family.
The first human he befriended was younger brother of Mr. Otis but as we've read that Mr. Otis rarely comes to the mansion is because in past— he struggled financially alot and making him torment his brother mentally and physically and later he died (idk maybe he did suicide??)
While he was on his last breath he asked demon to make his brother prosper financially but never let there be happiness in the family. Now the demon— to avenge his friend is tormenting the family.
Now people wonder why he favours MC alot that's because she looks more like his deceased friend.
Also a fact, the roses in the garden are connected to the demon aka Mr. Rogers ( i think more like lifeline something?).