The eldest son of the 81st Lord of Stormhold is hopeful about his chances of succession. His age shows, though: "Primus, the oldest, with white hairs in his thick brown beard, his nose aquiline, his eyes grey, looked expectant" (3.12). He chooses to wear "a long, black monkish robe" (4.141) in mourning for his father. In other words, Primus is a serious dude who means serious business.
He is also kind of paranoid, never eating or drinking anything that he doesn't provide himself. He pulls one on Septimus by renting a boat in Scaithe's Ebb but not being on it when it pulls out of shore. With his beard shaven, he looks like a totally different dude, and he sets off to search for the topaz-bearing star in his carriage. When Tristran helps him move a fallen tree branch blocking his carriage's path, he's reluctant to take on a passenger, but his runes give the okay, so he does.
Primus can't only read runes, though, he can also read ancient texts. When he sees the witch-queen's knives in the inn, he recognizes them: "There were tattered scrolls in the vaults of Stormhold in which those knives were pictured, and their names were given. They were old things, from the First Age of the world" (7.55). Primus is about to act on this knowledge when the witch-queen slits his throat, fating him to join his other dead brothers as a ghost. Bummer; he seemed like a decent dude.