However, people noticed that Sans said Toriel asked him to keep Kris away from chocolate because they'd eat all of them, establishing a connection to Chara's love of chocolate (also a piece of dialogue where if you choose 'NO' when the game asks you to smell the markers at the school, the narration says something like 'whatever, the chocolate ones smell fake anyway'). And things like hiding under Noelle's bed or pretending to be bleeding using ketchup and such are also Chara-like things.
So other than my 'Chara took over Kris in the epilogue', it can also be either Chara = Kris (directly or with alternate universe shenanigans), Chara took over the 'real' Kris like with Frisk since years ago, or they just share the same love for chocolate and macabre humor.
One thing for sure, it was Kris 'we' were controlling at the beginning since the save file said Kris before our first save over it.
There's also the little thing with having Kris eating the Choco Diamond giving him way more health akin to how giving Susie Hearts Donut gives her more HP ---
XBL GT: Demon Ninja X2 "There comes a time in every man's or woman's life where they have to make **** up to cover their ass." - Hazama