Board Thread:General Discussion/@comment-27130793-20160212011843/@comment-27620479-20160410172613

It's possible. The narration doesn't say who exactly are you saving. When you call out to whoever it is, Asriel is surprised. This, of course, could mean that they are surprised that you try to save them, but there's more.

At this point, memories come, starting with Chara, probably to indicate to be theirs. These memories are shown from third person perspective, instead of Asriel's. Also, they are seen as if they were in Frisk's head, and not Asriel's. We don't know if Frisk can enter Asriel's mind, but it is canon that they can experience Chara's memories as their own (as evidenced by the Game Over screen or the garbage dump scene).

Only after that is Asriel named. Now, as far as I know, they (as Flowey) recognize Chara's presence early on in the genocide route, but doesn't name them up to this point in the pacifist run. That would mean, that only after saving someone who is not named, does Asriel realize that Chara is present, and start to actually fight against them, even changing his attack pattern in the process. This is also the point where the menu options (presumably provided by Chara) become limited to saving Asriel.