Nikolai Rozhenko was a Human scientist in the 24th century. Nikolai was the only biological child of Sergey and Helena Rozhenko. In 2347, he became the adoptive older brother of a Klingon child, named Worf. The two went on to consider themselves brothers well into their adulthood despite their conflicting personalities.
Together the Rozhenko family was raised on the farm world of Gault, before eventually returning to Earth. Nikolai would accompany his father and brother on camping trips into the Ural Mountains. He was afraid of the howling of the wolves. (DS9: "Change of Heart")
Known for being somewhat of a loose cannon, Nikolai and his brother had many disagreements as they were growing up, even if they acknowledged and respected each other's strengths; Worf in particular noted that Nikolai was a strong-willed man and a natural leader. Nikolai attended Starfleet Academy but dropped out after one year because the rules were too stringent. (TNG: "Heart of Glory", "Homeward")
Years later, Doctor Rozhenko was assigned to the cultural observation post on the planet Boraal II and was the only observer present by 2370. He broke the Prime Directive by becoming much more involved with the Boraalans than necessary, although he was able to conceal his true origins from them. He conceived a child with a Boraalan female, Dobara, and became very protective of the Boraalans in her village.
It was at that time that Boraal II fell victim to a rapid atmospheric dissipation. Major storms hit the planet for several days before the atmosphere was finally destroyed, and Rozhenko led Dobara and the villagers into a cave to seek refuge after transmitting a distress call. The USS Enterprise-D responded to the distress call and Rozhenko pleaded with Captain Picard to save the small group of Boraalans. He felt it would be irrational to let the culture die and that the Prime Directive did not apply in this situation; not known at the time was the fact that Dobara was pregnant with his child, which served as a further motivation. Picard disagreed and did not permit the Boraalans to be brought aboard.
Shortly thereafter, the Enterprise-D witnessed the complete destruction of all life on Boraal II. However, Rozhenko used the systems interference from the atmospheric dissipation to transport a group of the Boraalans to the holodeck, recreating the caves in which they had sought refuge. Picard was infuriated at Rozhenko's gross violation of the Prime Directive, but decided to find a new home for the Boraalans. He assigned Dr. Beverly Crusher and Lieutenant Commander Data to the task of findng a new planet for the group.
They chose the isolated world of Vacca VI. While Rozhenko and Lieutenant Worf guided the Boraalans out of the simulated caves and to a simulated surface, Lieutenant Commander Geordi La Forge was able to gradually adjust the scenery to more resemble Vacca VI. Once the transformation was complete and the Enterprise-D reached Vacca VI, the Boraalans were transported to the surface, where they began to establish a new village. Rozhenko stayed behind with the Boraalans. All but one of the Boraalans were unaware of the move. (TNG: "Homeward")
Nikolai Rozhenko was played by actor Paul Sorvino.
Although the existence of Worf's adoptive brother was first established in "Heart of Glory," his name was never given until "Homeward." Prior to this, author Peter David used the name "Simon" for the character in the Starfleet Academy book Worf's First Adventure.
Since Russian tradition usually has a person bearing his/her father's surname as a middle name, Nikolai's full name might be Nikolai Sergeyevich Rozhenko, but this is not mentioned in any canon sources.