Quote from bakanimee
I found a manga called Happy Sugar Life. It's shoujo-ai and he's not the main yandere. He gets some decent screen time though.
Thanks! He hasn't quite crossed the line for me (could be due to the heroine stealing the thunder), but he does get quite.. insane.
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@Thouarttammy Thanks for the spoiler. I was already reading that series, so I'm quite excited now for the upcoming chapters. Too bad the translator is on hiatus.
I don't remember if I ever mentioned it, but one of the main guys from Vampire Sphere has yandere tendencies. The story has three main heroines and each of them have different love interests. My slight warning is that the names get very confusing and it's not helped by the characters looking very similar and that they seem to have different aliases/titles. Hence why I didn't write any names, because I can't remember them. I can't even tell the 3 heroines apart sometimes.
There's a side story (it seems to be an alternative universe from the main story, but same pairing) where he's a yandere in it. It's Vampire Sphere Extra, chapter 8. You could probably read it as a standalone.