Kamin was the identity given to Captain Jean-Luc Picard as an iron weaver on the planet Kataan in the 14th century when he was under the influence of an alien probe. It is unclear if Kamin was a real person who volunteered to have included in the probe or if he was fictional.
In 2368, the USS Enterprise-D encountered a probe launched from Kataan shortly before their sun went nova. The probe initiated a memory link with Picard and in less than 30 minutes he experienced a full 42 years of "Kamin's" life, including his marriage to and death of Eline, and the birth of their two children.
However, Picard's former life was not completely obscured during the memory link and he retained many of his former interests, such as astronomy and music. During the period as Kamin, he experienced the struggle to master the Ressikan flute, made observations of the stars from Kataan, and conducted studies of the soil and climate of the planet. By the time his grandson was born Kamin had concluded that Kataan was a dying planet. He lived just long enough to witness the launch of the probe that he would encounter aboard the Enterprise a millennium in the future. (TNG: "The Inner Light")
In the comic miniseries, Star Trek: The Next Generation - Perchance to Dream, the Enterprise crew were attacked by a telepathic weapon called the Chova, which forced its victims to experience dreams and hallucinations focused on their personal failures. However, it was discovered that those with Multiple Personality Disorder could render the Chova inert (the multiple personas overwhelming the Chova, which could only attack one personality at the time), so Picard was deliberately infected with the Chova, as his mind-meld with Sarek, the probe that gave him the memories of Kamin, and the remnants of Locutus of Borg still in his mind gave Picard the makings of an MPD. The four defeated the Chova, but Locutus then attempted to regain control of Picard's body, nearly 'killing' Kamin and Sarek before Picard gathered the mental strength to stop Locutus.
In the novel Greater than the Sum, it was because of the pain that Kamin felt at losing his son and daughter that Picard put off having children with Beverly Crusher for as long as he did.