Joachim Ziemssen is Hans Castorp's cousin: a young army officer confined to the Davos sanatorium, who thinks only of returning to his regiment, and whose illness is the reason for the visit that opens the novel.
When his cousin arrives, Joachim already speaks the language of the place: he says three weeks amount to almost nothing "up here," and at the first dinner he sums up the sanatorium in a dry sentence, "Here there is neither time nor life." He takes his temperature four times a day, and it is his heart that Hans Castorp first sees on the X-ray.
In chapter VI, tired of waiting, he leaves for the regiment with what the narrator calls a "half permission" and, at the station, calls his cousin by his first name for the first time, asking him to come soon too. He falls ill again in the regiment, returns to Davos, and dies in the sanatorium in the middle of a sentence about a request for an extension of his leave. Dr. Behrens sums up his death in military language: "He has slipped off to the field of honor."
Joachim goes down out of conviction and by his own choice: he goes to serve and comes back to die. Hans Castorp only goes down years later, and not by choice but because of the war; the cousin who wanted to be a soldier dies in the sanatorium, and it is the civilian who ends up on the battlefield. In chapter VII, Joachim still reappears at a séance, with the warrior's beard of his last days.