Transformations
Freeza and Coola's act of transforming from one form into another in order to tap into greater stores of his energy. In Freeza's first two transformations, his body simply changes size, shape, and appearance. His third transformation into his final form is a full metamorphosis, with his third form becoming as a shell or carapace and his final form exploding out of it.
None of Freeza's race are seen regressing back into earlier forms in either the Dragon Ball anime or manga. It is possible the transformation is one-way, though this is not likely since Freeza is spoken of having chosen his first (tiny) form in order to limit his immense power. In some games, Freeza mentions things that might confirm this like "It has been a long time since I had to take on this form".
Freeza is shown transforming through three different forms (2nd, 3rd, and 4th), while his brother, Coola, transforms into a fifth. Their father, King Cold, only appears in a form similar to Freeza's second form.
Cell has this same ability due to having some of Freeza and King Cold's DNA within him. However, he cannot simply transform as Freeza and Cooler can. He must first absorb the bio-matter of Jinzoningen #17 and #18 to advance from his Imperfect form to his Semi Perfect form and Perfect form. He is returned to his Semi-Perfect form after Jinzoningen #18 is removed from him by force, but thanks to his Saiyan and Namekian DNA coupled with a near-lethal injury, he is able to regenerate and advance into a Super Perfect form.
As henshin means "transformation", this technique can also refer to the Ozaru and Super Saiyan transformations of the Saiyans.
Character Meaning
変 (Hen) = Change
身 (Shin) = Person
SAIYANS
When facing dangers that would normally overcome normal opponents, some Saiyans have the ability to instantly increase their power dramaticly. It is unknown how the Saiyans developed this unusual natural ability or if there is ever a limit to how powerful a Saiyan can become, but so far twelve different variations have been discovered.
Many Saiyans live and die without ever achieving even the lowest of the Super Saiyan levels, but many would kill to have it. According to Saiyan records, it is beleived that a special bloodline carries the ability to change into a Super Saiyan, but that bloodline was dispersed many centuries ago. Freeza's best scientists theorized that the Super Saiyan bloodline was just the result of a random mutation, and if that is true, then other similar mutations could still occur.
FREEZA AND COOLA
Over time, Freeza's power became so great that his body could not comfortably contain it, and a series of physical transformations were developed to limit his actual strength. While other beings in Dragon Ball Z transform to increase their power, Freeza transforms to control and contain it. His true form is his fourth form, with each of his other transformations meant to restrict his power and conceal it from his enemies. Freeza even says upon transforming to his second state that he has a harder time controlling himself and his power due to the immensity of his current strength. That would suggest that the second and third forms are mere physical augmentations which would allow Freeza to build enough strength to revert to his true form. While changing shape from his "first-form", each alteration builds on the previous.
In all, Freeza demonstrated four transformed states, each with increasing power (some larger than others and each resulting in different physical attributes). Frieza may also use these transformations to purposely decrease his power in order to torment his opponents by giving them a false sense of hope.
Coola has an extra transformation that he learned to go beyond Freeza's. Originally, Coola merely told Goku that he had one more transformation than Freeza.
CELL
With each change of form Cell went through, his personality changed as well. Initially, Cell is single-minded, cautious, and sneaky. Upon reaching his first transformation, he becomes brash, impatient, and loses much (though not all) of his strategic tendencies. Upon reaching perfection, Cell displays a number of traits shared by those whose cells he possesses; Piccolo's cool intelligence and cunning, Vegeta's self-confidence, Goku's laid-back disposition, Freeza's conceitedness, and the Saiyan desire for battling strong opponents. Cell is also unique among Dragon Ball villains in that he has absolutely no desire for conquest or immortality. Another interesting characteristic possessed by Cell is the fact that he is very genuinely polite to his opponents (especially in the manga), complimenting them on their abilities and power when appropriate. Believing he is perfect, he merely desires entertainment, to enjoy his perfection, and after realizing his "true purpose": the annihilation of anything he considers imperfect, a category in which he places everyone and everything but himself.
BUU
The character with, easily, the most stages and transformations throughout the entire series. Absorbing one character after another (including himself!), Buu's forms were always amusing and fun to look at. Haow ever, Majin Buu's final form is Majuub.
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BEBI
One of the main villans of Dragon Ball GT. Bebi is single-minded in reviving the Tsfuru-jin race and exterminating the Saiyans, and over time it is shown that he has extreme cunning enough to dominate over the Z-Senshi, possessing Vegeta and successfully restoring the Tsfuru-jins' homeplanet. However, Bebi is also incredibly childish and immature; he is very prone to taunting, and flies into a rage if he is teased enough. Ironically, despite seeing the Saiyans as a barbaric race, he arguably finds more enjoyment in battle than they do; upon becoming a Golden Ozaru, Baby pretends as though he is in a mindless rampage (a trait common among all Ozarus and Golden Ozarus), and massacres countless followers as a means of testing out his abilities. He has a very intelligent, and "extremely egotistical and selfish" personality.
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FUSIONS
In the Dragon Ball universe, Fusion is the process of merging two separate beings into one, combining their attributes, from strength and speed to reflexes, intelligence and wisdom. When properly fused, the single being created has an astounding level of power, far beyond what either fusees would have had individually. So far, it has been shown that there are at least four different ways to fuse (three before Yo! Son Goku and His Friends Return!! was released).
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WHAT IF and OTHER
There are variety of fusions that are not seen above. However, they are from the what-ifs and Budokai Tenkaichi series.
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