Far from the powerhouse I remember from late-90s/early-2000s comics, but not as egregious as the Fate or Spectre from this set. Unlike those two, GL here actually does something interesting- free wall, as people are noting in this thread, is meaningfully different from free barrier. And as other people are noting, at only 35 points, he'll see a lot of ID Call-In use. He's no Jakeem Thunder, but I wouldn't hate seeing him in my booster.

Traded for this guy tonight and was pleased to discover the picture is not quite accurate- his Light Wall ClixFX marker doesn't use the "Energy/Special" mold (like Professor X and rare Magneto) as shown, it uses the actual "Wall" mold (like SR Iceman and Sandman). Much more sensible.

Regarding the Light Wall, are we to assume that this wall boxed in the character that occupies it or do we get to choose on which of the 4 sides (of the square) the wall is gonna be?

The second quote is from the power. You choose the side of the square that you want the wall to be on. If it was the way you said, I don't think you'd see a team without at least two of these guys on it.

Does his Light Wall barrier still -1 from those KC chases that can only be modified by other KC figures?

The modifier is coming from Alan Scott, who is not KC, so no.

Can the light wall marker be placed in hindering terrain?

I rarely got any offense while playing this figure, but his ability to taxi and apply -1 to opposing figures defense (often by placing his marker in an occupied square) was incredibly potent. Luckily the point cost of this figure is so low that it doesn't seem wasteful to include this GL only to do those two things.

Can you place the light wall in a square of a character in hindering terrain?

ABPIs001 Reality Gem has an amazing ability to make this character more like his comic book inspiration. Perplex TK and an extra bolt can really help out.