April 15, 2024


If you’ve already defeated Sans in Undertale , you’ve probably been as surprised as I was back in the day to see Sans bleed after receiving the final blow from Frisk on the Genocide route .

No wonder it surprises you since supposedly Sans, being a skeleton, shouldn’t bleed . This is one of the many reasons that make Sans such a mysterious being and that makes us raise an infinity of doubts.

There are quite a few Theories about it, from the most crazy, to others that could make a lot of sense and in this post I will explain all of them as well as my personal opinions and what I think is the real reason for this peculiar bleeding.


One of the solutions that have been proposed to explain why Sans, being a skeleton, can bleed is the Marrow .

The Marrow is basically that substance that is inside the bones, which also contains blood. That is why some have wanted to solve the mystery by saying that it is simply the blood of the marrow, but… this seems to me too weak an argument.

This makes sense of the fact that Sans bleeds, but not that he bleeds that much. I mean, Sans bleeds too much for it to be blood solely from the marrow, it is impossible for his marrow to contain that much blood and therefore, I would completely rule out this Theory.


Surely you already know the famous Theory that Sans could be a human , a Theory that as I have already discussed in this blog, I prefer not to go into too much detail.

Well, it turns out that this Theory is based a lot on the subject of Sans’s blood, since if he bleeds, he could be deceiving us and really be a human.

Of course, the Theory goes far beyond just blood, it has many more arguments in its favor, but here we are going to focus only on this topic.

By proxy, it could be that Sans is a human, but it seems too far-fetched to me. The blood is a very good reason to be in favor of this, but personally I am more in favor of believing that what we see is not blood, but something else closely related to Sans.


You already know Sans and his relationship with Ketchup . Sans not only has several stores where he sells Hot-Dogs, in which, incredibly, he seems to be in several stalls at the same time since we find him everywhere with his stall.

In addition, we know that he loves Ketchup, even in Undertale he makes us a joke of putting a few ingested amounts of Ketchup in our food when he invites us to eat (nice).

Well, taking into account his profession and his great fondness for Ketchup, it is not too far-fetched to think that the supposed blood is Ketchup. This in itself is already like a “meme” for many, the fact of saying that blood when it seems fake is Ketchup.

It is most likely that either Sans has a good assortment of Ketchups hidden between his clothes and that when they hit him they have broken. Or that it is even humorous (if it is so and in the situation it is in, applause for maintaining its humorous touch) it has decided to smear itself with Ketchup just as it receives the blow, something that given the powers it has , it would not be unlikely.

I am more in favor of the fact that he took them there and that when you cut the Ketchup it breaks and sprouts, the strange thing is that he even has Ketchup in his mouth… Could he have splashed it? In that aspect I do think it could be more of a joke on his part. But I find it hard to believe that in such a tricky situation he gets to make humor, even being Sans.


And to finish, what is surely the most meaningful Theory and that I am only going to summarize briefly since I want to treat it in depth in a complete post.

This Theory says that Sans is not a skeleton, nor a human, but that this is a «revenant» , which is a kind of skeleton that can even have parts with meat.

The curious thing about this Theory is how much it fits with Sans in many places and that is why it seems to me the most accurate of all the theories related to the “secret” identity of Sans that I have read. Besides, this perfectly explains the mysterious blood.

But, as I have already said, we will see this Theory later, so keep an eye on the  Undertale section , since sooner or later (I hope sooner) you will already have this Theory explained.

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