Hikaru Sulu was a secondary protagonist in the original Star Trek series and the various media based on the series.
Sulu was first introduced in the pilot episode Where No Man has Gone Before. At the time he served as the ship's physicist on the Enterprise under the command of James T. Kirk. Following the untimely death of Lee Kelso at the hands of Gary Mitchell, Sulu moved from science over to operations, and became the chief helmsman on the Enterprise.
Over the next five years Sulu served with distinction on the Enterprise, becoming very loyal to Captain Kirk. Sulu became close friends with many of his fellow officers on the Enterprise, including Spock, Leonard McCoy, Montgomery Scott, and Nyota Uhura. He was particularly close to Pavel Chekov, who sat next to him as navigator during the first five year mission.
Sulu would continue to serve under Kirk following the first five year mission. Seeing potential in the young man, Kirk arranged for his field promotion from Lieutenant to Lieutenant Commander. Seeing a strong leader in Sulu, Kirk encouraged him to pursue the command track, and had Sulu serve as his first officer when Spock was absent from the Enterprise.
A chance meeting with a woman named Susan Ling led to Sulu fathering a daughter named Demora Sulu. After Ling died of Sukaro's disease, Demora came to live with Sulu and eventually followed in his footsteps as a Starfleet officer.
By 2290 Sulu was promoted to Captain and named the commanding officer of the Excelsior. After a three year mission cataloguing gaseous anomalies in the Beta Quadrant he played a vital role in ensuring the success of the Khitomer Conference between the Federation and Klingon Empire.
After the Khitomer Conference Sulu continued to serve as the Captain of the Excelsior for many years, well into the 24th century.