How he would be as a yandere individual.

2p Denmark:

Magnus would be subtle at first. Doing some surveillance on her whenever he can and checking cameras when he is desk bound. This man knows how to be patient.
He will do a background check on anyone his girl talks to. If he does not like them, they have a tendency to dissappear.
Afterwards he will start randomly running into his dearest, making it seem like a coincidence and letting her feel more and more at ease with his presence.
This man is a great strategist, he will have everything planned and ready to go in any situation.

He will start a real personal relationship after his dear feels good with his presence, he will make sure she feels safe when he is around and manipulate the environment around her with his job to make her want to keep safe.

Two ways the next step could go, 1) he kidnaps her to keep her safe. 2) she comes to him for security and willingly agrees to stay.
Either way, he is getting her.
He will have everything ready, a nice room that is comfy and a private bathroom.
If she had to be kidnapped, though, He would probably lock the door to keep her in that room in complete safety.

He would keep her in complete comfort and safety to build a relationship with her that would be closer than before. He will eventually get her affection.
When he does, he will be very happy.
He is a protective type yandere who will do what it takes to win the affection of his love, he is tenacious and will win.

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2p Sweden:

Bernard would be the one to first start a relationship. As a friendly and chatty person, it is easy to win the friendship of his love. She won't notice the small changes as he will do what he can to keep her blissfully unaware of what he is doing.
Bernard will keep close to her on their outings, telling her about the recent crime increases in the area and how he is concerned about her safety.

He would invite her over regularly so she would be comfortable at his place, that makes having her stay after an arsonist 'attack' on her house easier. He will be a source of comfort in those times for her.
It would probably when she finds the meat cellar that she finally understands just what he was, but it would be too late at that point.

Bernard would have her already locked away as his perfect little love. He would keep her in his room and only give her the best.
It would take a bit before he would finally get her to come around to his way of thinking.
He then would make her his wife so she could never escape.

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2p Finland

Thurston would start out as a stalker. Heavy stalking.
Probably would meet his dearest at a gun event or she hired him once to be a bodyguard in a rough area. He would offer to keep being her bodyguard or offer to be a full time guard.
Most likely would pester her until she agreed. He is the stubborn sort that would.
He could follow her and build a relationship with that, along with learning more about the one he was becoming obsessed with.

Same as Bernard, he will try to make himself the sole comforting thing in her life to sneakily manipulate her. Even though he is known for his cold and impersonal dealings and interactions.
With how many enemies he has, his dearest would be in real danger and would need his protection. He would either stay with her or have her move with him into different hotels and apartments.
Finally after he gained enough trust from her, he would take her to his home.

Thurston would still have his hard outer personal but start to be softer towards his dear after he has her at his home for a while, either by gaining her full trust and love or by wearing her spirit down until she gives up fighting him.
He will take a challenge and will be much harder on the darling who fights back, probably resorting to some physical punishment or binding and isolation.

He will not be as rough once he finally has what he wants from her. Unless she likes that sort of thing. He will make it impossible for her to leave him. By imprisoning, marriage, chemical induced sickness, ect. She is his now.

Give up.

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2p Norway

Like Bernard, he would be the one to first approach his beloved to start a conversation to turn into a relationship.
He would seem a bit creepy with his burns but his friendliness would win people over soon. Loki would give his dear his number and call her to set up play dates on weekends so she and he could have some fun.
It would look like a pair of great friends at first, but keen eyed outsiders would notice that he would get mad if she was talking to another male or that he would always be looking at her.

After being friends for a little while, (y/n) would notice a string of murders and crime in her area. Police and agents would have no clue, her house would be broken into in broad daylight. None of the important things to her were taken.
Loki would offer her to stay at his house and helped move her stuff into his oddly large home.
Her house would mysteriously burn down and disappear afterwards, forcing her to stay with loki.

It would seem nice until she stumbled upon the things that were stolen from her home, pictures, some of her clothes and jewelry. 
It would be too late as loki had already put up a magic barrier and shackled his dearest into the house with even more magic, binding her into his home.
He would be kind to her and get her to love him slowly.
Or he just used magic to mess with her brain and force her to act like seeing loves him, he figures that soon enough she would eventually love him for real.

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2p Iceland

Subtle? Hell no. This young man will be all over his beloved, asking questions and giving gifts.
Say goodbye to privacy because he is everywhere.
Home? Nope, you live with him and have a new pretty anklet!
Need love? He has it for you!

Just give up.
There is nothing you could do.