So… I PROMISE I’m not one for giving characters unnecessarily tragic backstories, but I think there’s enough evidence to at least put this one forward. I also expect it’s been done before.
I think Kris was abused or neglected by their biological parents.
A few reasons, which aren’t much alone, but add up to a picture:
1. When they see the picture of unfamiliar humans, they ‘shut the book quickly.’ The fact the game specifies ‘unfamiliar’ shows Kris has no reason to dislike them personally. They just… don’t like seeing humans. Surprising, as you’d think the only human in a town full of monsters would feel less alone to see other members of their race.
2. Kris, like Susie and Lancer, is a very aggressive eater. We know they ate the entire pie Toriel made at least twice. We know that Toriel has to ‘lock the door’ to stop them from eating the chocolate (in fact, the description for Undyne’s box of chocolates suggests Kris would almost certainly eat them if they weren’t (possibly) being controlled by the player. Toriel even has to ‘lock the oven,’ suggesting Kris can’t even wait for pies to finish baking. We also know that Susie and Lancer don’t have great home lives.
Hoarding food is a known habit in people who were underfed as children. Kris still has a fear, even if they know it’s irrational, that one day, Toriel will stop feeding them.
3. Kris is very quiet and introverted. Again, that doesn’t say much on its own (I’m also an introvert) but there is some emphasis on this. With Frisk, supposedly they talked just as much as a regular person. You just couldn’t hear what they said. With Kris, they stay silent even when given direct questions by people who’re a danger to them. It suggests they’re a little closed off emotionally, though that could be making a jump to conclusion.
Kris also apparently dislikes the hugs Asgore gives them, though I can understand this (most teenagers probably wouldn’t) and they don’t react to Toriel’s hugs, so it probably doesn’t say much.
4. Kris barely acknowledges being bullied, even snickering in the face of someone as threatening as Susie, probably because they were used to much worse.
5. Their liking of knives (back to this later).
6. Kris was adopted by a policeman’s family. Again, says nothing on its own, but suggests there might be a reason WHY they were adopted rather than their family simply handed them to an orphanage.
7. Toriel's behaviour towards Kris - driving them the short distance to school and holding their hand while walking them in and hugging them - even though they're a teenager - seems a little overly protective.
7. The BIG ONE that collates them all: Kris is VERY thematically similar to CHARA. They’re in the same situation: a human adopted by a monster family, with Asriel as an adoptive brother and Asgore and Toriel as adoptive parents (assuming Asgore still has parental rights). They love chocolate. They use knives. They wear the same-coloured striped shirt. Chara hated humanity. Kris, given their reaction to that photo, also might.
And this video puts it very well how much evidence there is Chara was abused by their family and was likely the reason Chara went to Mount Ebott. Kris could have a similar history.
So, here’s my picture of Kris’ backstory: Asgore was called over as the nearest police officer to a human household where a young Kris was being mistreated. Kris’ family was arrested and Kris was put in an adoption home. A little while later, when Asriel was getting older, Toriel was thinking of another child, and Asgore remembered the human he'd saved, and suggested it to her, ultimately leading to their adoption.
This is getting pretty long, so I’ll make another post with the next part, which is kinda a different topic: how I think Kris likely knew Susie BEFORE Susie came to school, judging by Noelle’s blog. And I think Kris knew her FROM that adoption home.
It WILL make more sense when I make it, I promise.