During his time as a student in the Academy, Lee proved to have no talent for ninjutsu and genjutsu. As he was mocked by his peers because of this, Lee persevered, focusing on taijutsu.
When Lee eventually graduated he was added to Team Guy along with Neji Hyūga and Tenten. During the team's first meeting, Lee vowed to become a powerful ninja without using ninjutsu or genjutsu. Neji laughed at him, but their sensei, Might Guy, took a special interest in Lee. He encouraged Lee to keep at it and, in time, began teaching him powerful forms of taijutsu.
Lee is extremely energetic, single-minded, and over-enthusiastic. He is characterized as being the epitome of a "nice guy", having assumed many of Guy's personality quirks. One such trait is his apparent lack of common sense, believing at times that he can get what he wants by not wanting it. At other times, he takes copious notes, but forgets that he cannot check them during a fight. He has a keen sense of honour, and respects his opponents, refusing to hold grudges against those who have defeated him, and returning favours to those that have helped him. This was shown by his befriending of Gaara, forgiving him for crushing his arm and leg — injuries that nearly ended his career as a shinobi. Lee feels others should show the same respect towards their opponents, and is disgusted whenever someone is unnecessarily cruel to their enemies.
Lee is known for speaking respectfully to others, being very proper in both English and Japanese language versions, and never using contractions or swearwords. He also uses formal Japanese, applying proper titles ("-san", "-kun", etc.) to anyone he meets. However, Lee refers his team-mates Neji Hyūga and Tenten just by their given names, implying that he is close enough to them to use such an informal way to speak to them. His relationship with Guy has also caused him to acquire many of Guy's traits; in addition to his personality, his dedication to his promises, as well as his belief that one cannot dislike food, are characteristics of Guy. Lee also carries this small book and pencil to recall the various bits of information Guy has given him over time.
Lee's attempts to prove and better himself are recurring themes for his actions throughout the series. He is generally optimistic, and believes that, with hard work and passion, he can surpass a natural genius, but can be prone to depression when faced with setbacks. His drive to succeed was initially most evident with his rivalry with Neji, having learned from Guy that having a rival will provide incentive to succeed. Neji, a "genius" ninja, found Lee's efforts of bettering himself to be in vain, and believed that Lee would be a "failure" for the rest of his life.
As such, Lee resolved to prove Neji wrong, and confronted another "genius", Sasuke Uchiha, in an attempt to test his worth. While Lee maintained the desire to defeat Neji in a fight throughout the Chūnin Exam Arc, after Neji's eventual defeat by Naruto Uzumaki, another so-called "failure", Lee became less interested in this goal. Neji's rejection of his previous belief in fate also contributed to this, as he now respects Lee, believing that he will surpass the gifted with enough work.
Shortly after his introduction to Team 7, Lee revealed that he liked Sakura Haruno, asking her to be his girlfriend and vowing to protect her immediately after they met. While she did not seem to share these feelings for him, and considered him weird, Lee maintained these feelings, and went out of his way to help her throughout the Chūnin Exams. Sakura became more appreciative after this, and punches Naruto whenever he insults Lee.
Despite this, Sakura does not seem to return any romantic feelings for Lee, as she refers to him with the more formal "Lee-san", as opposed to "Lee-kun". After the Exams' conclusion, Lee's interest in Sakura went largely unaddressed in the manga. It is occasionally referenced in the anime, such as when Naruto fought Lee in the Lee's Dōjō Arc; Naruto transformed into Sakura, causing Lee to forget about the fight long enough for Naruto to attack him.
Lee after opening the chakra gate:
Lee using Front Lotus on Gaara:
Info from: http://naruto.wikia.com/wiki/