Theory of how Kenpachi and Yachiru are connected.
I am convinced that Yachiru is Nozarashi. By extension, Kenpachi just might go bankai before the end of the war. Now, I know we've seen this theory before, but I don't think I've seen one thread with all of this evidence gathered together, and there were still people who said that it wasn't possible or made no sense. Okay, so Yachiru mentions that before Kenpachi came along, she "had no name", "had no parents", and "had never seen anything in colors other than the bloody red". http://img.mangastream.to/manga/bleach/ 114/%5Bmanga-rain%5Dbleach-ch114-16.png
"Here's the chapter/page where Oetsu says Kenpachi stole his zanpakuto from a dead guy: http://www.mangapanda.com/bleach/529/5" Thanks avernii!
He was immensely powerful, but untrained, hence the weapon being in shikai, yet looking like nothing more than a chipped nodachi. Then he fights Unohana and limits himself. Because he limited himself, it took him years of slaughtering with his zanpakuto to build his power back up to captain level (which explains the time-skip between Kidpachi fighting Unohana and young adult Zaraki meeting baby Yachiru). He finally built himself up to be captain level, even while limited. His love for battle made him closer to his zanpakuto than he ever acknowledged, which causes him to fulfill the first step of bankai; forcing the spirit of the zanpakuto to manifest in the physical world. Nozarashi manifested as a young girl who always has some sort of skull/bones motif (who, in her memories, thinks of herself exposed, able to be stepped on at any time [Google: "Nozarashi, meaning "exposed in a field", is a motif of skulls, bones, and/or skeletons"]).
Yachiru represents an innocence that Kidpachi never had, a youth Zaraki never experienced. When he finally meets her, he asks her where she came from. Her response is to touch Nozarashi. He asks her if she's scared of the sword, and she doesn't react; because she is the sword. Then, he asks her name. http://www.mangapanda.com/bleach/577/14
When he names her, he doesn't know her name is Nozarashi because of his whole "I rely on my power only" schtick, and so he calls her Yachiru to honor Unohana. If I recall (again, going off hazy memory here), when Ichigo is training with Urahara to learn Zangetsu's name, I thought there were times he heard Zangetsu speak, but the "sound" would cut out when he said his name, until Ichigo was ready to learn the name Zangetsu. This would explain why Kenpachi could go so long conversing with Yachiru, with her being able to speak and act like a regular shinigami. Yachiru is to Nozarashi what Old Man is to Zangetsu, except Ichigo is quicker on the uptake to learn the true name of his spirit (yes, I know Old Man Zangetsu is actually young Yuhabach and Ichigo's latent Quincy powers, but for the purposes of this conversation that isn't relevant).
Now, recall Yumichika; he calls his zanpakuto "Wisteria Peacock", knowing that it hates the color wisteria, so that the only release it demonstrates is multiple blades. When he finally calls it "Azure Peacock", it does its full release and allows him to access its Kido-type abilities. Well, Nozarashi knows that Kenpachi is too stubborn to learn it's name, so he cannot use its full power, but the name he does use is not an insult, but an honor. Yumichika calling his zanpakuto wisteria causes it to go from sealed, to minor release. Kenpachi consistently acknowledging his zanpakuto as Yachiru allows it to be absurdly sharp and strong, despite constantly chipping. The chapters where Kenpachi shatters the meteor and kills Gremmy, the absolute last we see of Yachiru is her watching Kenpachi releasing his zanpakuto, calling out "Drink, Nozarashi", and then she's gone. After Gremmy dies, when Kenpachi is looking for Yachiru, he calls out her name, and Nozarashi (Bleach wiki: Nozarashi [野晒, Weather-Beaten One]) goes back to looking like a weather-beaten nodachi. Then, he finds her clothes and lieutenant badge, just laying abandoned on the ground, and not a single person in the area senses her reiatsu. He starts yelling for his division soldiers to look for her, and the rain coming down looks like tears; he finally realized who Yachiru was, and where she now is. What's more than this, look at her shikai compared to the released Nozarashi. She has a big skeleton wearing black (Nozarashi is a large black sword) with a long white bone structure in its face (just as Nozarashi has a long white edge). She also has a large, fluffy creature (whereas Nozarashi has a large, fluffy tassel). http://img2.wikia.nocookie.net/__cb20140417055411/bleach/en/ images/d/dc/577Nozarashi.png http://1.bp.blogspot.com/-2vm99euV8xk/UxeXTRSgLeI/ AAAAAAAADd8/L1ghgmFO6KU/s1600/Yachiru%27s-shikai.png
The nail in the coffin is Yachiru's shikai itself. Isane freaks out over it; "This isn't a katana that can transform into a living being, like Captain Unohana's. And it's not made of a living material, like Captain Hitsugaya's... The katana hasn't transformed..." Also, Yachiru cuts Guenael with her shikai, then she calls it's name. It is already known that only captain level shinigami who have achieved bankai can use their shikai without calling it's name. The only zanpakuto similar is Komamura's, but his only summons a full being in bankai, not two of them in shikai. The best explanation is that she isn't using shikai; she is the shikai. The large skeleton with the dagger, the fuzzball with the cleaver, and Yachiru with her katana, are all aspects of Nozarashi.
With all the power ups everyone is getting, Kubo knew that even getting his shikai wouldn't be enough to keep the Kenpachi--the strongest shinigami--from being outclassed by the others. Since there's no time for him to actually master kendo, it is obvious Kenpachi would need to eventually hit bankai. So Kubo had to consider, "How can I explain Kenpachi gaining his full shikai, and then so rapidly gaining bankai?" This could be that answer.
Now, you point to her zanpakuto and ask "how". Again, all of this is speculation and theory, with bits of evidence that could possibly support it. Tenken, when an arm, has a sword. Kokujō Tengen Myō'ō has a sword as well. Yachiru never actually released her shikai, Oetsu never said she had a real zanpakuto, and shikais/bankais can have their own weapons. Just speculation, but perhaps Sanpo Kenjū is an ability. Think about Ryujin Jakka. Ryujin Jakka specializes in flame, and as shikai, Yamajii can use Jōkaku Enjō, a prison of fire; Taimatsu, a simple inferno that can be as hot as Yamajii wants and attacks only what he specifies; and Ennetsu Jigoku a trap of an inferno of hellfire to consume all within. Kenpachi's zanpakuto specializes in cutting. It can cut everything. Well, he of course can cut anything he hits by brute strength. But if he is supposed to be able to cut anything, what about the things he can't hit? Maybe Sanpo Kenjū is one of Kenpachi's shikai special abilites--his Getsuga Tenshō if you will. Old man Zangetsu could use Getsuga Tenshō, right? So Yachiru/Nozarashi would be able to use Sanpo Kenjū, which not only explains why she never actually releases her shikai, but it also makes more sense as Sanpo Kenjū as an ability functions similar to Tenken, rather than being an insanely unique and unprecedented release.