Shirosaki is his INSTINCT which has turned into a living form, which ichigo needs to get a control of to become his ultimate "himself"..
in other words, when people got their soul chain cut off, they're instinct turns into a soul linked being with the original soul of the human, but only if the human finds his shinigami powers. the hollow is in other words the "evil" side of ichigo, which i don't really find interesting to use in bleach, we have not yet seen shirosaki making comment on like destroying everything. he just wants to take over the soul to become the ultimate himself, since he misses the human part that ichigo has. and by shirosaki's words, he said you can use my powers until i find a chance to crush your skull (because he now is cornered into the edge of the soul and has less ways to take over until ichigo is near death probably).
as for the vaizards, they used techniques to artificially surpress theyre souls into transforming slowly into a hollow, but they had the power to resist that, so the hollow is in the same position as shirosaki, but in my theory.. i think their inner hollows are different to shirosaki, since shinigami are dead human.. they don't posses the instincts they had as a human anymore, their instincts of when they were living beings has dissapeared, and thus making it easyer to handle their inner hollows, since they didnt understand why it took so long for ichigo to master just 11 seconds.. so in other words .. vaizards don't have to deal with the instict that shirosaki has..
A human posseses instincts like a animal, since practically humans are highly evolved animals.. because instinct is the nature of any living being. which hollows and shinigami's obviously do have but it's different from living beings. this also explains why ichigo learns so fast in some sorta way. let me explain this.. humans and animals have a part of their instinct which allows them to evolve and adjust theyre conditions by experiences, but due the fact humans have developed their own thoughts above their instincts while evolving, they now miss the ability of instincts since they adjust themselves by thinking how to change..
but.. ichigo was somewhat of a special person which had the adjustment instinct running at full power.. but, didn't had his natural killing instincts even running a little bit.. which he now seems to be taking control over since he actually is controlling shirosaki which i think is his battle instinct..
(the adjustment instinct adjusts his powers and reactions with any fighting experience).. just like animals do..
if you look at it this way.. you can definetly see why aizen wants ichigo on his side, or perhaps dead.. because ichigo is becoming the ultimate entity too fast.
that's it lol, sorry if some sentences were confusing.
EDIT: the instincts of adjustment like i wrote about, is what i think, the hollows and shinigamis dont have anymore since they died as humans and lost that part of instinct, only the battle instinct remains in theyre soul
EDIT: shirosaki is trying to take over ichigo's soul because normally, who wouldn't want to take over the one in lead and just like animals battle their own group for the lead of the territory, ichigo and shirosaki battle to take over the lead theyre territory AKA the soul.
next is from another thread in the bleach section about uruhara's bankai
i now have another theory based on this..
Shinigami represent a part of the instinct which actually is fighting..
Hollows represent the instinct of survival..
Human represent the insticts of adjustment by experiences..
if a human is able to get its hands on shinigami powers like ichigo.. then the effect is a shinigami which's powers raise dramatically fast.. because of the fighting and adjustment instincts.. for example look at the short time that ichigo got this powerfull.
if a hollow get's it's hands on shinigami powers/or they are maintaining powers growing to shinigami powers like the arrancar.. then their survival and fighting instict raises to dramatically hights as you take the espada as an example.. this also counts for shinigami that obtain hollow powers
if a human maintains both shinigami and hollow powers.. then the three instincts combine and make it the ultimate being..