Knuxsu didn't just happen, snap, like that. It all started in Knuckles the Echidna issue #4.

How Knuckles and Julie-Su met:
Here's how it happened. Vector the crocodile was in the pond or lake or type of body of water swimming when he sensed a threat and came to the surface to see a Dark Legionnaire. He thought it'd be real bright to attack the Legionnaire from behind (not his brightest idea). the Legionnaire quickly defeated him.

Later on that same day Knuckles was with his mentor, Archimedese, meditating and, of course, not focusing. The Dark Legionnaire came up from behind and attacked Knuckles. Knuckles fought back and when he got on top of her he took off the mask only to find out that the Legionnaire he thought to be a male turned out to be a female. A female none other than Julie-Su.

Now at that time Julie-Su had already left the Dark Legion but to the Chaotix she was still known as a prime suspect. She joins their group but it takes a while for them to trust her. Vector takes the longest to get well adjusted to their new, female, team mate.

How Knuxsu began:
It wasn't until an early issue of Knuckles the Echidna that Knuckles and Julie-Su had their first kiss. Well...kinda. Julie-Su pecked Knuckles on the cheek and Knuckles said sourly that he didn't exactly like her and Julie-Su recalls that she was simply returning the favor.

In Knuckles the Echidna issues #26-28 was the plan for Knuckles' sixteenth birthday. Julie-Su had already developed feelings towards Knuckles by that time and made Knuckles jealous that he arranged a date for the two of them only to be interrupted by his surprise birthday party. Julie-Su wanted to impress Knuckles so she actually wore a dress (which isn't a very common aspect of Julie-Su). When Knuckles said how he has never had something like this in a long time Julie-Su claims that it's his night and they can just pose pone the date until next week. Later on into the party Julie-Su and Knuckles do what is called the soul touch, where they embrace in a kiss becoming each others soul mate.

Knuxsu today:
Ever since the soul touch Julie-Su and Knuckles have grown toward each other in a way that know one else has. They truly love each other and unlike regular couples (sorry if i offend the other Sonic couples) they stay true to each other and although they may fight they always recover after a short time. I recall they only fought once.

Julie-Su is more protective of Knuckles. Although they use to be enemies she cares for his protection more than anyone else. She was willing to let herself die just to save Knuckles from becoming the second Enerjak. Julie-Su may not be the average girl but when it comes to Knuckles' safety she can get pretty sensitive. Especially when she found out that Knuckles had become Enerjak against his own will.

Knuckles and Julie-Su are barely apart. They fight by each others side. They stick together through thick and thin because they know that no matter what, they'll always find a way to make it through because they're love is stronger than anything in the world and nothing, nothing at all, can break their bond and love for each other because it will always remain strong.