Two crazy killers, two psychopaths, however who'd actually win in a one on one fight. There will be two parts to this. The first will be Jeff the killer from just his origin story vs Yandere chan without her Easter eggs just her mane weapons and abilities. The second round will be expanded universe Jeff vs Easter Egg Yandere Chan.
This of course won't have things such as attack potency (except for round 2) because of the fact that these two aren't powerful because of how much they can destroy, they're just pretty strong people after all. We will however be going over their weapons, strength, fighting speed and durability.

First let's go over weapons as this is a fun one. Jeff the killer has his knife that's all he has, meanwhile Yandere Chan can use three weapons at a time.

While she has many to select from we'll be sticking to this limitation, besides if she spent too much time changing weapons the fight would be over right there, so let's give her her katakana to start off with.

With this alone Yandere Chan has a clear advantage as she can keep her distance, meanwhile Jeff would need to get up close.

Now let's go to her Knife, another weapon she's commonly show with. Of course knife vs knife there's no weapons advantage here.

Now for her baseball bat (these will be the three weapons Yandere will be using) while a baseball bat is a strong weapon it is kind of slow. If Jeff did one good dodge Yandere would be done for.

But this category isn't over yet, while Yandere Chan has the quantity Jeff has the quality. He's been shown capable of using his knife way more affectively than Yandere Chan can use any of her weapons.

So this one will be a tie.

Next let's go to experience which is where things get a bit tricky. We are lead to believe that Yandere Chan is active for a month to possibly even three months in the game due to how many rivals there are to eliminate.

Meanwhile for Jeff.... We don't know, at the beginning of the story we see that he's made a very big name for himself with his kill count. So we'll just say a few weeks to a month to be on the safe side (if they ever mention in the story how long he's been active please tell me and this will be changed)

Next up we go to speed.

Fighting speed to be more specific.

This one is a tricky one as Yandere Chan has study points.

These different points boost each of her categories. This makes it hard to pin point her speed (especially since I don't have the game) the same goes for Jeff, however he was able to react to gun fire something Yandere Chan has never done, also a teacher can react to her attacks so she's not like some incredible athlete.

Now for endurance... Jeff wins. Sorry but Jeff wins in this category, there's no way I can give it to Yandere, I'd have to make something up to give it to Yandere.

Conclusion

This would be a good fight, however when you look further into it it gets clearer and clearer. Yandere normally uses her prep time, a way to plan and manipulate others, sure if you want you can go around and slaughter people but that's players choice, that's like saying Mario is evil because in super Mario 64 if you want you can throw a baby penguin off a ledge, or goku's an idiot cause you can make him just fly into a wall in xenoverse 2.

Yandere Chan takes a more stealth approach with time and manipulating, while Jeff would just kill because he wants to, he's more prone to take drastic actions and is more aggressive and seems to be more of a risk taker from what I've seen, he's taken out adults and high schoolers alike just like Yandere Chan however in the end these are two kinds of monsters.

However I'm not saying this would be an easy win, Yandere does have multiple weapons and could possibly outsmart Jeff.

Winner: Jeff the killer