Cedric as a yandere would be delusional, soft, clingy, protective and obsessive over the love of his life aka. you. He would always spend time with you no matter what and even if his schedule is packed as hell, he'll still find some time to make for you. For him, not seeing you at least 5 times a day is equivalent to him not breathing. And he won't be able to sleep till he doesn't strike up at least one conversation with you after every class is finished lol

He's polite, charming, handsome and you'd actually be lucky to have this goody two shoes here as a yandere for you. He never wants to see you sad or upset or even have tears in those wonderful eyes of yours for that matter. If you're upset and crying about something it just breaks his heart and he'll start hating himself since he couldn't do anything to protect you and you're suffering because of him. But beware once he does find out the reason as to why you're crying though, I can't see him ACTUALLY murdering someone but he'll put them under the Imperius curse and make them say sorry to you, even if it's Malfoy

The worst this goody two shoes here can do is go as far as manipulation that's it. And using the Imperius curse, but other than that he's not insanely possessive over you when someone talks to you like some other yanderes I know (I'm looking at you ferret boy)

He's also a BIG TIME worshipper for you, he literally worships the ground you walk on. He thinks you're a literal divine angel sent from the heavens above to bless him and grace him with your wonderful presence. You're perfection in his eyes, he can't help but want you all for himself. Of course, he'll understand why other people could also be infatuated with you, you're amazing and so...precious. He literally radiates softness, warmth and love when he's around you, lol I make him sound like the male version of Cupid

As much as he won't ever do anything without your consent, he loves holding your hands, an indication and signal for everyone out there to know that you belong to him and only him, no one else, loves hugging and cuddling with you and will also kiss your hand like a true and fine gentleman. And if you initiate any signs of physical affection with him, he'll feel like he's died and gone to heaven and he'll feel so flustered yet happy at the same time, his heart about to burst from his chest. His darling has finally responded to his love, and now it's time for him to take the next step

Now, he has the idea of asking you out planned very carefully. In case you guys forgot, let me remind you that he's the Golden boy of the Hufflepuff house. Meaning his house is willing to do whatever they can to make him happy and content even if it means getting Cedric closer to you and asking you out no matter which house you're in. I guess you could also say that they might also come off as platonic yanderes for you too since they're going to be helping Cedric planning on how to ask you out. Now, he'll ask you out in a public setting with people around and fancy and nice decorations and stuff like that so you won't be able to reject him. Yep, under that innocent good boy personality of his lies a dark manipulative personality of his

Though he might never hurt you he'll turn out to be a bit scary after a while and sooner or later, he's going to get paranoid about your safety. He's also a paranoid yandere, worrying about your safety constantly and he'll kidnap you faster than you can say 'Merlin's pants'. He can't bring himself to punish you at all, he's too soft with you but don't take his kindness for granted else he'll slip Amorentia in your food/ drink or put you under the Imperius Curse to be in love with him. He'll make sure to spoil you with his love and affection forever and ever and ever till time ends...