Why Asriel WON'T be in Deltarune*

*Unless he's the Roaring Knight.

So, I don't think that Asriel will be in Deltarune. Why? Well;

In chapter 1, Toriel says Asriel will be back from college "next week". The wording and when he'll be back in that next week is pretty vague, but one could assume that she means one week from now. Considering that it's "the weekend" during Chapter 4 as stated by Susie, and there was school in Chapter 2, Chapter 1 must have been a Thursday.

Chapter 1 takes place on Thursday, Chapter 2 on Friday, Chapter 3 on Friday Night/Saturday Morning, and Chapter 4 all throughout Saturday. Considering there's no school in Chapter 5, it must take place on a Sunday. So! Obviously, chapter 6 will take place on a Monday.

With all of that out of the way, if the "next week" was referring to Sunday for the festival... Asriel wasn't there. At least, we didn't see him if he was. Toriel might mention him in Chapter 6. But for the sake of there being no confirmation, he wasn't at the festival. So clearly, his opportunity to visit them during the festival is gone.

Now, why can't he show up? Unless Chapter 7 is a major time skip to the NEXT WEEKEND, why would the golden student Asriel ditch college? The obvious answer is the Roaring, but it's described as an apocalyptic event. Any normal town (hometown is clearly not normal) would be locked down and hiding or evacuated. Asriel COULD run home, but if you saw massive monsters in your town would you bother going back? You'd think that your family also did the sane thing and evacuated.

And yeah, that's why I don't think Asriel is going to show up unless he's the Roaring Knight