Ronin was an anaphasic lifeform. He was known to have existed since at least 1647, when he took up residence with Jessel Howard shortly after appearing in the city of Glasgow on Earth. He lived with every subsequent Howard daughter until 2370, when he was destroyed by Beverly Crusher. In corporeal form, he took the image of a male Human approximately thirty-four years old. He was able to maintain a strong grip on his hosts, causing them to react and behave unusually.
Ronin used a candle in order to maintain molecular cohesion. He needed a corporeal lifeform to take physical form. In the 1600s, he realized that Jessel Howard had a compatible biochemistry. He lived with her until she died, and then took up residence with her daughter. This continued for twenty generations. Ronin moved with the Howard family from Scotland to North America and eventually out to space. By the mid 24th century, he was living with Felisa Howard at the Caldos colony. Ronin did not reveal his corporeal form to anyone other than his host, leading many (such as Ned Quint) to believe he was a genuine ghost. Felisa wrote detailed descriptions of her experiences with Ronin in her journals, and he made her very happy. Felisa never mentioned Ronin to Beverly or anyone else, however, in order to keep his secret.
When Felisa died in 2370, Ronin moved on to her granddaughter, Beverly Crusher. He appeared to her at Felisa's funeral, throwing a camellia on Felisa's grave, and later filled her grave and house with camellias. Ronin seduced Beverly, and his hold on her was quite powerful. While he was in the process of merging with Beverly, he maintained cohesion with help from the colony's weather modification network, causing systems disruptions and killing Quint, who was attempting to stop Ronin.
After Beverly resigned from Starfleet in order to move to Caldos, USS Enterprise-D Captain Jean-Luc Picard became suspicious. He wished to meet Ronin, who no one else seemed to have noticed. Ronin showed himself, hoping to drive Picard away. The captain then began bombarding him with questions as to how he came to Caldos II, and subsequently received a communication from Data and Geordi La Forge that they had detected anaphasic energy on Felisa's body. Ronin threatened to go to the governor to stop them from exhuming the body, but Picard pointed out that the governor would have the same questions that he did. Ronin then attacked the captain, went to Felisa's grave, reanimated her body and attacked the two Starfleet officers. This was the breaking point for Beverly, and she realized Ronin was a malevolent leech who had used her entire family for generations. She destroyed the candle and then Ronin himself after he attempted to bond with her. (TNG: "Sub Rosa")