Where did Beast end up while looking into the mirror?

The way the different personalities change is depicted as something like a meeting where they sit around and when one of them 'grabs the light', which resembles taking over the consciousness of the human body.

The Beast, the 24th personality, showed up on stage to show all the personalities how powerful he can be so Patricia and Dennis convince the other personalities that they're not defects, but some coalition of extraordinary power. He can come back any time if needed. The personalities were originally in conflict because the Beast doesn't sit around a table like the rest of them.

This is confirmed when doctor Fletcher is telling us what she's heard of what Dennis and Patricia believe.

DR. FLETCHER: Do you still have strong beliefs?
DENNIS: That depends on what.
DR. FLETCHER: This story of The Beast. One thing, Dennis, that may comfort you if you are confused is that you've met the other alters. You're all in a room in chairs, right?
DENNIS: Yeah.
DR. FLETCHER: But you never met The Beast. Because he doesn't reside with the rest of you. Because he resides in the train yard, as the story goes, because Kevin's dad left on a train. But the fact is, you and Patricia have never met The Beast.

(Of course, Dennis says later that he lied and he has in fact seen the Beast, but the how of that seems irrelevant and pure speculation)