Dunno. Zarla calls both of them "human" although I'm not sure if she only calls Chara "Fallen Child" or not, since Frisk technically fell too...Plus it varies how many stripes zarla draws their shirts in.Considering the bright stripes and the fact that this one's face is not shown, I think it might be Chara though.Zarla never draws Chara's face but almost always draws Frisk's face.Chara's canon face is also always hidden until the very end of the Genocide run where they have asmiley face and blushy cheeks. Followed by a creepy face with black/red empty eyes and a wide smile, very creepypasta-esque, except it's canon and official. But all other times Chara is shown, their face is hidden or facing away from the screen.This is even the case of Kris, the Deltarune version of Chara. Their face is never shown until the very end of the game/chapter 1, which is...well... a creepy, murdery, Chara face.TLDR: I think it's Chara.