If a character with Hypersonic Speed is surrounded by opposing characters, would that character still be able to break away after succeeding a break away roll?

X = clear terrain
O = Opposing character
H = Character with Hypersonic Speed

X O X
O H O
X O X

If by "blank" you mean clear terrain, sure.

Characters are not blocking terrain (or any other kind of terrain). While the HSS character can't enter the square occupied by those characters, absolutely nothing prevents the HSS from moving through the intersections between them.

I see. For some odd reason I was thinking that the two opposing characters were creating a "wall" effect that would block movement out of that square.

Thank you!

They would block LoF from being drawn between the intersection, but not movement or adjacency.

The first sometimes makes people think that the others happen as well.

Common misconception but easily made. Easier to think of this eventuality in terms of range or close combat. As the diagonal square is one of 8 adjacent squares to H, they can move accordingly but as previously stated for LoF, the two X pieces create a 'wall' of sorts.

Please don't do that. Stuff like this is the whole reason there are misconceptions in the first place. As you say, this is a common misconception that a lot of people seem to have and I would bet a fairly large sum of money that for most of those people, the misconception stems from trying to equate the situation to stuff like ranged combat.

So please don't think in terms of range or close combat. Think in terms of movement. Because that's what this is: movement. Movement has different rules than making attacks.

I just had the LoF rules stuck in my head. I'll equate this only to movement. No big deal. (It was ruled correctly anyways, so no worries! I just didn't want to screw over another player.)

As long as you were both having fun, I don't think it really matters either

Since when does fun have anything to do with it?

You're right. Sorry. I forgot that was a house rule