A yandere girl in manhwa is often extremely possessive and obsessive about the person she likes. She can go to extreme lengths to keep them close, sometimes even resorting to violence or manipulation.
In manhwa, yandere boys typically have a dual personality. They might seem normal on the surface but go to great lengths to keep the object of their affection close. Their actions can range from overly protective to outright violent when they feel threatened.
I'd recommend 'Twisted Love'. The yandere male lead's actions keep you on the edge of your seat. The plot has lots of twists and turns that make it stand out. The way the relationship develops is quite unique.
There's also 'The Yandere Male Kagome's Quest'. This one is different as it focuses more on the male Kagome's journey while still maintaining his yandere traits. He goes on adventures, all the while being possessive and overprotective of the people he cares about. It's a unique take on the yandere concept in a fanfiction setting.
Look for fanfic communities dedicated to the source material that Phobos and Will are from. There, you can ask for recommendations of 'yandere phobos x will fanfic'. Members are usually quite helpful in sharing their favorite stories in this genre.
Well, 'That Time I Got Reincarnated as a Slime' can be considered in this regard. Rimuru, the main character, experiences an isekai adventure, and there are some characters with possessive or extreme loyalty traits that could be loosely related to yandere. Also, 'Log Horizon' is an isekai novel where the characters' relationships and their growth in the new world might have some yandere - like undertones at times.
One popular 'yandere x kagome fanfic' is 'Obsessed Love'. It has an interesting plot where the yandere character shows extreme possessiveness towards Kagome in a rather unique Inuyasha - inspired setting.