Cell

Cell (セル, Seru) is one of the major super-villains of the Dragon Ball manga and the Dragon Ball Z anime, and is the ultimate creation of Dr. Gero, designed to have all the abilities of the greatest fighters to have ever inhabited or visited Earth. The result was a "perfect warrior", possessing numerous genetic traits and special abilities. It should be worth noting that he is the only Red Ribbon Android to not have the Red Ribbon insignia on him at one point or another. It is also worth noting that he is one of the few Androids not directly created by Dr. Gero; the only others are Android 13, Android 14, Android 15, and possibly Android 8. Both Cell and Androids 13, 14 and 15's conceptions involved Dr. Gero's Super Computer.

Cell's personality changes drastically throughout the Androids Arc. He is unique among most villains of the series, being that he is quite sophisticated. As his evolution proceeds, he can be observed as having a more patient and calculating approach towards his goals. Because of his genetic composition from other warriors, he is able to psychologically manipulate those warriors and exploit their weaknesses to his advantage.

At first, Cell's desire to complete his evolution by absorbing both Android 17 and Android 18 was what fueled him. Upon reaching his final form, his eagerness to test the limits of his newfound power was what defined his character.
With each transformation, Cell's personality underwent a drastic change. Initially, Cell is completely single-minded in pursuit of his goals. Upon reaching his first transformation, he becomes far more brash and impulsive in his actions. Upon reaching perfection, Cell displays a number of traits shared by those whose cells he possesses; Piccolo's cunning, Vegeta's pride, Goku's laid-back disposition, Frieza's smugness, and the Saiyan lust for battle. Perhaps Cell's most distinguishable trait in this form is his uninhibited vanity which he shamelessly puts on display by launching the Cell Games, a "tournament" organized for the sole purpose of showing off his newfound power. It can also be seen during Cell's confrontation with Gohan when he affirms his "true purpose": the annihilation of anything he considers imperfect, a category in which he places everyone and everything but himself.

In the English manga, Cell is always referred to as an "it", while in the anime (and the Japanese versions of both), he is referred to as "he." He is likely described that way in the English manga to emphasize the fact that he is an artificial being.

All about Cell :
1. Larval

2. Cocoon

 

3. Imperfect Cell (不完全体セル, Fukanzentai Seru) or 1st form Cell (第ー形態セル Dai Ichi Keitai Seru) is Cell's mature form prior to his absorption of Androids 17 and 18.

4. Semi-Perfect Cell (半完全体セル, Hankanzentai Seru) or 2nd Form Cell (第二形態セル Dai Ni Keitai Seru) is a form Cell achieved after absorbing Android 17

5.  Perfect Cell (完全体セル, Kanzentai Seru) is the ultimate form of Cell, which he assumed after absorbing both Androids 17 and 18. 

Information from:
http://dragonball.wikia.com/