Jun's most important goal in coming to the World of the Living this time is Ichigo Kurosaki; everyone else is just an afterthought. Jun Junichi what will become of Aizen in the end, and Aizen Hirako have no chance of getting involved.
How powerful were Aizen's fourth and fifth evolutions? It's hard to say. The original work says it surpassed the limits of Soul Reaper, but it didn't really feel that way; it was just so-so. After all, the omniscient and the Almighty Yhwach appeared later on.
Let's just discuss one question: Is the Captain-Commander's Aizen capable of defeating Aizen in his fourth evolution? I think it is, though it might not be able to defeat Aizen in his fifth evolution.
Since Gin Ichimaru can defeat someone in their second evolution, and his third evolution is at least on Just Below Chief Captain-level, if Unohana were willing to take action, she and Zaraki could probably defeat someone in their third evolution.
Then, during the fourth evolution, both of them were powerless. At this point, Yamamoto Genryūsai should still have a fighting chance. After the fifth evolution, he reached a new level.
These are all just speculations at the moment. The Final Getsuga Tenshō, is the ultimate technique of the Kurosaki Shiba lineage. The effect of this move is to use all of the Soul Reaper's power to kill the opponent. Please note! The Shiba Clan guards the gates of the Soul King Palace!
Guarding the gates doesn't seem like a very prestigious job. Compared to the Kuchiki Clan, whose commanders hold positions in the Soul King Army, it's incredibly low-class. But usually, those who guard the gates of the royal palace are the imperial guards.
The Shiba Clan actually held the position of Imperial Guards, while the Zero Division belonged to the Eastern and Western Depots, which were separate systems. Imperial Guards were all top-tier experts, so the difference between the Shiba Clan and Kuchiki Clan lay here: one excelled in team battles, while the other was invincible in one-on-one combat.
However, as we all know, there are very few invincible imperial guards. Those who are truly invincible would have become great heroes or great villains long ago. Who would want to be an imperial guard? Moreover, the imperial guard's duty is to protect the emperor, so each imperial guard has a deadly move!
This decisive move is crucial; its significance lies in the message: "I'll risk my life to take you away!" Protecting the Emperor? As long as the intruder is taken away, that's protecting the Emperor. One's own life is unimportant. A true imperial bodyguard must have this understanding.
The Shiba Clan has been doing this for so many years, and this is the origin of the Final Getsuga Tenshō technique. After using this technique, the Soul Reaper will lose the concept of Soul Reaper. The explanation is rather complicated, but simply put, using it means death.
After using it, even the soul is absorbed into this move—a move that throws everything away, using the soul itself as a catalyst to obtain a higher level of striking power. Normally, the Soul Reaper would be finished after using this move.
Ichigo is special, which is why he survived after using the Ichigo, and even later regained his Soul Reaper powers. This is all due to Ichigo Kurosaki's uniqueness, which is also why Isshin Kurosaki also knows this technique, doesn't use it.
If Isshin Kurosaki were to rise to the top, he could defeat Aizen, but the price would be that Ichigo, Yuzu, Karin would become orphans, and this timeline would also include the little brother Maya.
However, in this timeline, Rimi is still alive, so Jun doesn't care whether Ichigo can learn this technique. Because if Isshin didn't teach Ichigo this move, then Isshin's intention was to do it himself—this is Jun's last trump card.
Therefore, Junichi isn't worried about Ichigo's growth at all, since Aizen is destined to lose. As for whether Aizen will pursue a higher level and undergo a sixth evolution, Jun currently has reservations. Jun's guess is that Aizen will never reach a higher level, because there are always trade-offs.
This concept of a higher level could very well change Aizen, making him no longer Aizen. Aizen's goal is to change Soul Society, to have a god descend upon Soul Society, but not to become a true god. This distinction is crucial. Aizen will not become God, even if God's level is Explosion high.
In pure speculation, this is the real reason why Aizen ultimately abandoned the Hōgyoku: because the ultimate evolution of the Hōgyoku wasn't what Aizen needed. Is the Hōgyoku good? Of course it is!
But it's like when a free card for the best gender reassignment surgery in Thailand is placed in front of you, and they repeatedly guarantee that it will transform you into the most beautiful person of the opposite sex. Would you do it? It's the best gender reassignment surgery in the world! But everyone understands...
With no need to worry about Aizen, Jun focused all his attention on Ichigo. He had nowhere to live in the World of the Living anyway, so he decided to stay with Hirako.
What could Hirako do? However, Hirako also knew that Jun's goal was Ichigo, so after recording the Arrancar's information, he went to find Urahara. Hirako didn't yet have the means to contact Soul Society; Soul Society didn't trust him.
With nothing to do, I picked up a magazine and read for a while when Isuzu came looking for me.
"Chun~ will swim with me~"
"Can you swim?"
"No way, you've never taken me swimming."
Chunzhen has never taken Isuzu swimming before. She's practically a superhero; she can't even drown underwater. What's the point of swimming? But if my darling wants to swim, then let her swim! As long as my love is happy.
Want to go swimming but don't have a swimsuit? Well, this problem was solved by Kageyama Akira! This young lady has a peculiar hobby: collecting swimsuits, and all sizes at that, from ultra-thin AA bras to enormous N-cups—you can find them all on her collection.
So, both Isuzu's C and Nel's G were satisfied, which was quite impressive. However, Jun didn't have swim trunks and didn't want to wear Ōtoribashi's, so she just lay down by the pool with Ōtoribashi and enjoyed the beautiful sight of the beautiful women playing in the water.
Swimming, eating, playing games—the three of them came to have fun. Around 4 PM, Ichigo Kurosaki finally arrived, Rukia, Sado, and Uryū. They were all training with Hirako.
"Yo~ Hoo~", Chun waved and greeted them as soon as they entered from the recliner.
Because Ōtoribashi usually lay there, but today Ōtoribashi went out to run errands and was just lying there, but since there was someone there, they didn't pay much attention.
"!!!"
"How did you get here!?" Ichigo was dumbfounded, and the others weren't much better off.
Chun chuckled, "Why can't I be here?"
"You"
Chun put down her magazine, stood up, and greeted them, "I came here specifically for you, little one. Didn't you say you had a problem? Come on, let me see what the problem is."
Ichigo hesitated, as if he hadn't seen it before. Only after the group arrived at the Training Grounds did Ichigo say, "That hasn't happened since I came back."
"Oh~?" Jun wasn't too surprised. Ichigo's amazing quality was that he always unleashed his full potential in dire situations, all sorts of different kinds. He wouldn't have it unless it was a desperate situation. "Tell me the specifics of this Hollowfication Mask Fusion. I need to know to the Visionary."
"Oh, I was fighting Nnoitra at the time, Nnoitra was one of them."
"Don't bother talking to me about Nnoitra, I know his abilities better than you do," Jun interrupted Ichigo's explanation directly, "Get to the point."
"Oh~, after he Resurrección," Ichigo glanced at Jun with concern. Jun looked quite helpless, so Ichigo quickly said, "Okay, it's just that after he Resurrección, all four of his arms used their power to knock me to the ground, and then he used Cero, and that Cero was incredibly strong!"
"That's Gran Rey Cero; Arrancar of Vasto Lorde rank or higher has one," Chun explained.
"Oh, it's Gran Rey Cero. That Cero was too powerful. I had already withstood it once before, and it was simply too much for me. My Zangetsu was about to break, so if I had tried to withstand it back then, I would have been dead. Suddenly, I opened my mouth and spat out my Cero."
Chun snapped his fingers. "So, you only use it in dire situations, facing a Cero that's guaranteed to kill you, right?"
"Uh, I don't know either," Ichigo scratched his head.
"Haha, your abilities are indeed unreliable. Come on, let's fight—no, let's have a real battle! Watch out, I really will kill you, the serious kind!"
Ichigo swallowed hard, then his morale soared. "Bring it on! I'll see just how strong you really are!"
They entered in their spirit forms. Jun rolled up his sleeves, and it was then that Ichigo noticed that Jun was wearing two bracelets on his left arm. One of them, a short, single-strand bracelet, was something his father, Isshin Kurosaki, also had, but the long bracelet had seven rows of beads.
Saying you won't bring any equipment is a joke; you can't go without it!
"Sanctification, Super Roar, Siphon, Super Condensation, Diamond Strength, Joint Defense, Life-Taking Scythe, come forth!"
Six beads instantly appeared on Chun's body. Sanctification was the main armor; Super Roar added energy rings and jet nozzles to the gauntlets; Siphon added Siphon to the shoulders and skirt armor; Super Condensation was added to the back; Diamond Strength was a body state type; and Joint Defense was armor linkage.
The effect of Super Roar is to condense energy attacks like Cero; Siphon absorbs Reishi; Super Condensation increases Reiatsu; Diamond Strength increases one's own strength and defense; Joint Defense unifies the defense values of all equipment and then distributes them as needed.
Just like its name, the Life-Taking Scythe's power lies in its ability to absorb the life force of an enemy after inflicting damage. The larger and more numerous the wound, the faster the absorption.
While Jun was equipping himself, he explained the abilities of those equips to Ichigo, leaving the four of them stunned, especially Rukia.
"These are all Zanpakuto?"
"Yes, they're essentially the same. Zanpakuto, or Spirit-Soul Orbs, are no different, are they?"
"But why are there so many?"
"Who told you that each person can only have one Zanpakuto?"
Rukia couldn't accept it. "Zanpakuto is the manifestation of each person's soul! It's a unique power! This is common knowledge in Soul Society!"
"Hehe, how about this then?" Jun unleashed his full power, violently Siphon the Reishi in the Training Ground with his siphon technique. A burst of light condensed in Jun's hand, forming a sword, which Jun then tossed to Rukia.
Rukia felt a familiar sensation from the sword and glanced at the one at her waist.
"Do you dare to Shikai this sword?"
Rukia suddenly knelt down, because she knew that the sword in her hand was also the Sode no Shirayuki.
"Alright, alright, forget about everything else. Ichigo, remember this: you're not from Soul Society. Let's fight!"