In comic #16, "Reunions", Julie-Su wants to support Knuckles when he's feeling down and sits alone on the pier gazing into water. She comes to him and kisses on the cheek. Knuckles is really surprised and looks angry, he asks "What did you do that for?", and she replies "I was simply returning you the favor". He asks her not to do that again and she says that she wouldn't even consider it. However, both of them thinks something else in their minds.
Julie-Su: "Oh, yes I would--I wish I understood why!"
Knuckles: "It really wasn't that bad! Actually, that felt kinda nice!"
(I wish I could get my hands on that comic book, you can't buy it so easily in Latvia)
However, nothing particular happens between Knuckles and Julie-Su for a long time, if I miss nothing, but issues ##26-28 are dedicated to their relationship. Both of them feel something towards each other but they hide their feelings; Julie-Su goes out with some other guy once but still she thinks about Knuckles, Knuckles sees her in the cafe with that guy but eventually he braces himself and asks Julie-Su if she would like to go out with him, and she agrees! However that Friday is Knuckles' 16th birthday so there's a surprise party organised for him. At that party Knuckles and Julie-Su finally have a real kiss on the lips.
sadly, Julie-Su does not appear in other series.
if Julie is her name, suffix "Su" means that her mother had the same suffix and her children would have it too, it is something similar to human second names.