Mass was just a function of one's impact on the space around them, and by extension, gravity. Mass was constant while weight fluctuated depending on the g forces acting on a body. How much a Mass warped space decided its gravity.
Speed impacted Mass as well. As speed approached that of light, Mass approached infinity.
The faster one wanted to go, the more pressures weighed one down. This was why it usually required breaking laws of reality, or rather having the strength to, that allowed one to break these kind of rules and exhibit strength and speed beyond reason.
However, there were other methods as well.
What Ryu had realized, however, was that he could control space such that his mass had a smaller and smaller effect on it. At the same time, his Northern Heavenly Wind also had the capability of making him lighter and lighter. By stacking these effects, he could limit the heaviness of his body and his higher and higher speeds.
Then there was shrinking.
When Ryu usually used his Spacetime Soul Nature, what he was doing was shrinking the earth beneath his feet. This was done by taking two spatial nodes and bringing them closer together. This was a weaker form of "teleportation" as compared to entering and crossing the void, but it was still exceptionally powerful.
However, when this earth shrinking was done, all of the space in between would increase in density, this likewise increased drag and slowed Ryu considerably. In addition, such an action was essentially stretching his body out, increasing his surface area. Not only did this increase drag even further, but it also increased the acting forces on his body. While his mass was the same, because he covered a larger area, he was far more fragile.
For example, a piece of paper could survive a gust of wind... But what if that same piece of paper was stretched, its mass and material remaining the same, until it was ten times larger? A hundred times? A thousand times? Before you even reached that point, a small gust of wind which couldn't even harm a fly would shred it to pieces.
After Ryu comprehended these two concepts, utilizing his Spacetime Soul Nature and his Northern Heavenly Wind to counter them instead of just passively accepting them caused his speed to increase by leaps and bounds. Even as he ran, he only seemed to continue accelerating despite the fact his effort hadn't increased.
Ryu's comprehension was simply on another level. Just with a slight quiver of the Quivering Spectral Wings, he had learned enough to practically be untouchable beneath the Sky God Realm and Sixth Heaven in speed.
As for where Ryu was going with this speed, though, even he wasn't 100% certain. The first thing he had to do without a doubt was to target the largest city he could without being exposed. He had some tiny understanding of the layout of the Fourth Heaven, so he used it to his advantage, quickly making it out of the two Sect's territory.
Ryu was cautious as he entered the wilderness. This was the Fourth Heaven, not the First Heaven. There would be God Beasts in this place and it would be a shame if he got himself killed strolling into the wrong one's territory.
He wasn't sure how effective the Friend of Beasts aspect of his Bone Structure was currently, but he preferred to not learn the hard way.
Soon, Ryu made it to a perfect city. It took him not even a day. This city, although he didn't know the name, was massive enough to have the information and resources he might need, but tiny enough that it wouldn't be on anyone's radar.
Ryu slipped inside after paying the entrance fee.
He didn't immediately go to the stores to exchange for the materials he would need. He would only do that at the end when he was certain that it would be his last task here. Him doing such a thing would be bound to attract attention.
This time, Ryu had shrouded his cultivation level and his face, but given the climate, this likely wouldn't be enough, so he didn't rely too heavily on it.
Ryu went to a few places and found maps that were more and more detailed.
He didn't plan on heading to the Fifth Heaven immediately. Before that, he wanted to comprehend the Fourth Heaven in its entirety.
Ryu seemed to have a lot of skills, so it was exceptionally easy to forget... But his greatest skill, between even the both of his lives, wasn't his comprehension skills, nor his determination, or even his various talents and Daos...
His greatest skill, if he had to name one, was as a Ruin Master. If there was anyone who could find the opportunities resting in this Fourth Heaven, it was him, and the best part was that he knew exactly what to look for.
He adjusted his requirements based on the current combat strength of the geniuses that should be at this level currently. Then he lined them up with what he slowly cobbled together.
In the end, he even chose to leave the metropolis he was in without buying what he needed and go to another, then another. The ideas rolling around in his head continued to increase. Time passed and Ryu didn't even seem to notice that an entire 30 days had passed without him doing anything.
But then, his gaze suddenly shone with an exceptionally dazzling light.
"... Yes, an Immortal Sakura..."
The words drifted into his ears. This conversation was happening over a hundred kilometers away, but Ryu's ability with his Northern Heavenly Wind by this point had increased to the point that even this was nothing more than a joke.
The instant he heard Immortal Sakura, his gaze became like a blinding torch in the night.
He had heard about Eska and Isemeine on the First Heaven, but it seemed that he had finally found their traces once again.