Watch Out (Psycho-thrillers) ⊂(ʘ‿ʘ✿)

With all his memories wiped out, Jesse, wakes up in the hospital and in front of him stands a man that suggests he's Jesse's lover. However, the only fragmented memory Jesse has of that man is him trying to kill Jesse. But is his fragile memory reliable? Who's telling the truth and what is actually going on? Why was he in the hospital to begin with?

a mystery/puzzle-game, not scary
It’s Christmas Eve, and Doyoung’s made a reservation at a luxurious hotel in the hopes of spending a romantic day with his crush, Hyunjin. But nothing seems to be going according to plan. They lose cell service, miss their reservation, and drive through a terrible rainstorm only to arrive at a haunted-looking hotel. When Doyoung asks for a refund, the imposing concierge insists he must participate in a game—a game that quickly turns Christmas into a nightmare.

serial killer
"The person who is destined to be the perfect lover is secretly a murderous devil?! There are many secrets to the widowed mother. The life of Qiqi is completely changed by a mysterious stalker."

Things got way more interesting when the familiars were introduced, now it's actually cool action/gore with an eerie aura so just hold out until that.
This manga would've been 10/10 would recommend to my friends if it didn't have children get involved with adults sexually (no sex but there's a kiss and nude scenes) because the emotions are so raw and powerful, the gory art's insanely cool, and the characters (specifically souichi and osamu) felt really detailed and complex.