WARNING: The following contains spoilers for Bleach's, "Thousand-Year Blood War Arc," by Tite Kubo, available now in English via Viz Media.
As is the case with most shonen anime, no one's powers were cooler than the protagonist's, in this case, Ichigo Kurosaki's from Bleach. Spoilers are ahead for anime-only watchers, so be warned. Ichigo was way more than he seemed to be at the beginning of the series. Ichigo was revealed to actually be the son of a Soul Reaper captain and a full-blooded Quincy, the latter of whom held the Hollow known as White within them. This led to their first child Ichigo being a human, a Soul Reaper, a Quincy, and even part Hollow hybrid, making him a truly unique being.
Acting as the representation of his Soul Reaper powers, as well as the base for all of his other powers, Ichigo's Zanpakuto, Zangetsu, also represented his ever-growing understanding of his powers and true nature. Throughout the series, Zangestu kept changing in shape. Every time that this happened though, Ichigo himself also underwent some kind of change or gained a greater understanding of his potential. For example, the sword he used at the start of the series was never seen again after he unlocked his Shikai. This was because the spiritual energy he was using was mainly Rukia's, with his own barely leaking out just enough to influence the sword's size.
The first time readers/viewers got a true glimpse of Ichigo's actual powers was when he awakened his own Soul Reaper powers and was almost turned into a Hollow. While the almost-Hollowfication was explained away at this point it was actually the first moment the series hinted at Ichigo's true nature. This was because the awakening of his Soul Reaper abilities was quite different than every other case.
All Soul Reapers from the Soul Society are given Zanpakuto when join they the ranks of the organization. These specific Zanpakuto are known as Asauchi and have been forged by Oetsu Nimaiya personally. These Zanpakuto have no real special abilities when they are given to their respective Soul Reapers, as the Soul Reapers must imprint their own souls and power onto the blade. This is why there are no duplicates when it comes to Zanpakuto, as each one has its own name, powers, and even spiritual energy. But how did Ichigo do this if he never had an Asauchi?
Asauchi are forged by melding together the souls of multiple Soul Reapers together into the form of a blade. Coincidentally, this is how Sosuke Aizen created the Hollow known as White that infected Ichigo's mother and was later passed on to Ichigo himself. When Ichigo awakened his Soul Reaper powers, rather than imprint his power and soul onto an Asauchi, he did so to the Hollow within him, turning that Hollow into his Zanpakuto spirit. This is why his inner Hollow looked like a completely white version of Ichigo.
For most of the series, Ichigo is led to believe that Old Man Zangetsu is his Zanpakuto spirit, not White. This was all a lie fabricated by the Old Man spirit, which was in fact the manifestation of his Quincy powers. Knowing that if Ichigo became a Soul Reaper it would mean later going against Ywhach, the Old Man did everything he could to restrain those powers and become the dominant spirit within Ichigo. He only started relenting when he realized Ichigo would die without access to more of his Soul Reaper and Hollow powers.
At first, Ichigo only had access to his Shikai, which while incredibly strong, was only a fraction of his true power. In this state, Zangetsu looked like a giant khyber knife. Ichigo's spiritual energy in this state was bright blue, similar to that of a Quincy's which could be an indicator of the Old Man's interference. Even in this Shikai state, the inner Hollow was able to intervene on Ichigo's behalf here and there, mainly by either training him inside Ichigo's "inner world" or by creating Hollow Masks as a form of armor to protect him.
Ichigo's original Bankai was probably the biggest hint as to the connection between his Soul Reaper and his Hollow powers. In this form, Ichigo's sword becomes a long, black katana, which looks suspiciously similar to White's own blades when it attacked Isshin and Masaki, Ichigo's parents. Ichigo's spiritual energy became pitch-black with a blood-red outline around it, which according to Ulquiorra felt closer to that of a Hollow's energy than that of a Soul Reaper's. He also gained a cloak that resembled the Old Man's and became significantly faster and stronger than he was before.
If that wasn't enough, it was only in Bankai that White Ichigo would try to take control. For a Soul Reaper to be able to perform Bankai, they must have an incredibly strong connection to their Zanpakuto and be able to draw out its full power. In Ichigo's case, due to his Zanpakuto being a Hollow, this would mean that he would need to draw out even more of White Ichigo's power and create a stronger connection between the two. This probably made it much easier for Ichigo's inner Hollow to exert control over him, which could be seen during his fight with Byakuya.
This influence and fight for control would become one of the biggest plot points of Bleach during the Arrancar arc. During this arc, Ichigo not only came to an understanding with his spirit but also further mastered his Hollow-based powers by being able to draw out more of them. Ichigo would grow a Hollow mask over his face while his blade stayed the same. The understanding they came to was that Ichigo would get to keep control of his body, but once he failed then his inner Hollow would get to take over.
This was retrospectively an obvious foreshadowing to when Ichigo basically transformed into a full Hollow. While in this form, Ichigo was stronger than ever before, having full access to his Hollow powers as well as his Soul Reaper ones. In many ways, this might have been the truest manifestation of his Bankai at that moment. Given that his Hollow was his Zanpakuto, their connection was probably the strongest it had been up until that point.
Interestingly enough, it was that connection that actually saved his powers. After his final battle with Aizen, Ichigo eventually lost all of his powers and ended up learning Fullbring as a way to bring them back. Coincidentally Fullbring was a power developed by humans that relied on Hollow spiritual energy. Children of mothers that had survived Hollow attacks were able to draw out the souls and power from inanimate objects, especially ones they have a connection to. He realized that by accessing his Hollow powers, he should be able to reawaken his Soul Reaper ones due to their connection to one another.
In Ichigo's case, he did this with his Substitute Soul Reaper badge which had been with him since the beginning of his journey and even contained a large amount of his power. With a little help from Rukia, who used her own Soul Reaper powers to jumpstart the vestiges of what remained of his own, he was able to regain his powers but now stronger than ever due to his stronger connection to his Hollow powers.
After mastering Fullbring and regaining his Soul Reaper powers, Zangetsu was a whole different kind of monster. In both Shikai and Bankai, the blade had become bigger, with his Shikai getting even larger than before and his Bankai getting longer with unique grooves on the top, now even more closely resembling White's original arm blades. At this point, it seemed like Ichigo was as strong as he was while using the Hollow mask without ever having to actually wear it.
However, even after all that Ichigo still hadn't unlocked his true powers and didn't even have a real Zanpakuto. It was only after he lost devastatingly to Ywhach and went to have his Bankai reforged that he learned that he never even had a real Zanpkuto. It was around then that he learned his true origins, as well as his parents' that made him fully understand what exactly he was and what his powers could be.
One of the main reasons his Hollow and Soul Reaper powers were so closely linked was due to his parents. In order to prevent Ichigo's mother, a Quincy named Masaki Kurosaki, from Hollowfying after she was infected by White, Isshin Shiba, Ichigo's father, created a soul chain between his Soul Reaper powers and the Hollow inside her. This caused Isshin to lose his Soul Reaper powers, but it also permanently linked the Hollow to them. This made the Soul Reaper powers inherited by Ichigo the same as his Hollow powers, while separate from his Quincy ones.
This revelation led to a meeting within his mind/soul with the Old Man while Nimaiya was forging his Zanpakuto, who finally revealed that he had been lying to Ichigo the entire time. He both revealed himself to be a manifestation of his Quincy powers and that this whole time Ichigo's inner Hollow was the true Zangetsu. Strangely enough, Ichigo realized that the Old Man was incorrect, and instead of gaining one Zanpakuto he ended up with two, with the larger one representing his Hollow Powers and the smaller one being his Quincy ones. Ichigo's Soul Reaper powers acted as the base that linked the three together by having them each become Zanpakuto.
Now not only could Ichigo draw on both sets of powers and use them as blades, but he could also even go into a Hollowfied state while maintaining complete control of his body. In this form, he'd gain one horn on the left side of his head as well as Hollow markings, with his larger sword turning snow white. In Bankai, the swords would merge, both as a way of signifying the unity and balance between his powers, with the smaller sword becoming a hard white shell over the larger blade, that becomes identical to his original Shikai.
Both this form and Bankai resemble the state that the Zangetsu spirits took when they merged together while training Ichigo in the Dangai before his final fight with Aizen. This merged form of both spirits trained Ichigo to achieve his full power as a Soul Reaper, making him stronger than even Aizen after merging with Hogyoku. Therefore, it makes sense that together they help Ichigo become the strongest he could ever be, with power even greater than Ywhach's.
It might be weird to imagine that the Old Man was wrong, but it was more so that he didn't understand what he was any better than Ichigo once did. No Quincy has ever had a manifestation of their powers in the form of a spirit like Ichigo did, which already proved that there was something different about the Old Man. Beyond that, the Old Man might have limited Ichigo's power, but he also helped train him much of the time, further showing that he was just as much of a Zanpakuto spirit as White was. Even his new Bankai represents this, as the Quincy blade wraps around the Hollow one, enhancing it instead of suppressing it. Ichigo finally managed to find the inner peace both his Zanpakuto spirits wanted, giving him full control of the power that was his birthright.
