As is evident from looking at him, Kirby is a small pink spheroid being who possesses two red foot-like appendages for walking, two stubby arms for grasping (more on that later), two oval-shaped eyes with dilated glossy pupils and blue irises, a lipless mouth which can dilate wildly, and two blush-marks on his cheeks.
Kirby is eight inches tall (in his main series), which is about as tall as the average human head, and while his weight has never been formally given, he likely does not weigh very much, since he can hover by holding in air and flapping his arms. His body is extremely elastic, able to contort both inward and outward in every direction, suggesting that he has no rigid skeleton. Despite this, he defaults to his spherical form, which suggests some sort of supportive structure which prefers that shape.
Movement and Form:
Kirby is bipedal, which means that he walks on two feet. Unlike the rest of his body, his feet are hard and rigid, much like a pair of shoes would be. (it is still within the realm of possibility that Kirby's red feet are in fact just shoes, but all game sources so far have referred to them as his feet, and he has never been seen without them outside of certain transformations)
When walking, Kirby's feet appear to move on their own, independent of the rest of his body, yet they carry him forward. It may be that his feet slide across his skin, or that his skin is too smooth and uniform for us to notice it moving and stretching along with his feet. Either way, it is not entirely certain how the musculature of such movement works.
When hovering in the air, Kirby depends on the propulsion of his arms to move about. Naturally, this is much slower than his feet would move him, having less to push on. Unlike his feet, Kirby's arms are extremely elastic, capable of stretching not only outwards, but also up and down his body. He is able to meet the ends of his arms both below and above his face, as well as behind him, which is more than can be said for less flexible humans.
Kirby has no fingers, yet he is still able to grip things, and quite firmly. My prevailing theory is that Kirby's hands are coated in numerous microscopic suction cups that all work in tandem. I say this because it seems, he often doesn't have his hands folded over or around what he holds onto. Often, he's just touching it.
Much like a bird of prey, Kirby seems to be able to lift things that are heavier than himself. However, he is not able to hover while holding such things, nor can he toss them very far (without the aid of a Copy Ability). It is likely that, were he real, Kirby would not be able to lift as much as the average human can.
In addition, while Kirby is able to fit a great many things into his mouth, he cannot hover while his mouth is full. He has to spit out or swallow first to get rid of the weight (more on that later).
Kirby's eyes appear to be analogous to human eyes in most every respect. The only real difference is the size of them in proportion to his face/body. They take on an oval look under normal circumstances, and appear to be only glossy pupils and a portion of the iris, but when Kirby is startled, the rest of his eye becomes visible.
(just in case you thought this was just in Kirby 64)
Despite not having ears, Kirby does appear to be able to hear just fine. Additionally, he seems to be most sensitive to hearing on the upper sides of his head/body, analogous to where ears would be on us. I say this because that's how he wears his Mike hat.
As such, I suspect Kirby does have hearing apparatus. It's just hidden under the skin, and probably not very far in. It's likely that he would not like being flicked on the head in those places.
It is not clear what exactly allows Kirby to Inhale (much less copy abilities). Some have speculated, especially since Triple Deluxe, that Kirby's stomach is a veritable black hole, which draws things in at Kirby's command. Unfortunately for them, black holes in reality don't actually draw things in on their own, at least not any more than anything else with gravity would.
For a little background understanding, human lungs work because muscular contractions change the volume of the lungs, thus changing the air pressure. Doing so creates a pressure imbalance, which causes air to flow from areas of higher concentration to lower in order to equalize. So when the lungs expand, the air pressure is less inside them, drawing in air from outside, and when they contract, the air is pushed out again.
None of the above paragraph seems to apply to Kirby, since his Inhale is continuous, and doesn't seem to require him to exhale at the end of it. This suggests that something is at play inside of Kirby which defies terran biology. He may well be harboring a wormhole or a pocket universe inside of him which is never satisfied, much like a black hole, but it doesn't consume Kirby as well. This of course is pure speculation, and probably always will be.
Kirby has a voracious appetite, and while he can be satisfied, he always seems to have room for more. He has a tongue, and is clearly able to taste, given that he has clear preferences in foodstuffs, but he has no teeth, nor a clear throat. When anything enters his mouth, it seems to be immediately broken down inside him, unless it is a living thing or Star Block, in which case it is either spat out without issue (i.e. Kirby's friends in Return To Dream Land), or transmuted into a spitting star. Naturally, this spitting star can also simply be absorbed by swallowing it.
Given Kirby's seemingly endless capacity for storage, it seems unlikely that he would need to excrete waste in the same way that we do. Instead, whatever star stuff is left over from what he's eaten brushes off of him when he moves, jumps, and lands. Overall, a much more efficient and cleaner model of digestion than ours. Kirby's metabolism is such that he can heal very quickly from food he's eaten, but on that same note, he does appear to need to eat on a much more frequent basis than we do, and in much greater quantities.
Kirby does appear to need to breathe to survive. Since he has no nose, this explains why his default expression has his mouth ever so slightly open. Yet, he is often seen in space and underwater doing just fine. It could be that, for the former, the space around Pop Star is much more aerated than ours, and for the latter, Kirby may just be really good at holding his breath. (in Mass Attack, the mini-Kirbys were not able to hold their breath underwater indefinitely, which may suggest that Kirby's ability to Inhale may also allow him to store air inside him)
It is unclear if Kirby's Inhale is tied to his regular breathing or not. If it isn't, his lung capacity is likely quite small in comparison to his stomach capacity. Naturally, his high-pitched vocalizations would be tied to this as well.
It is unclear how Kirby's body works to nourish itself once he's taken in food. It can be speculated, however, that he does have blood, as evidenced by his blush, which does expand to cover more of his face when he's hurt, or experiencing certain emotions. This cannot be totally confirmed, however, since outside of certain dark matter eyeballs, blood is never spilt in the Kirby series.
Homeostasis & Perspiration:
Kirby is likely warm-blooded, and while he can feel chilly or overheated, he has an incredible tolerance to both heat and cold, as demonstrated in his many adventures.
Kirby can and does sweat, though this only seems to be apparent when he is tired during a fight. This suggests that he does tend to build up heat which needs to be released through his skin.
Kirby is clearly capable of feeling pain, as he cries out whenever he is hit. He also definitely has some sort of brain, though where it is inside him is debatable. It could be in his feet for all we know.
This is an area that we simply know nothing about. I have my own theory about how Kirby's species propagates, but it's nothing more than fan-fiction and head-canon.
Let's just say a wizard did it and move on.