GW has a plan for the Lost Primarchs (and I think I've figured out part of it)

[Karhedron]

There was one place where one of the Primarchs (possibly Fulgrim) recalls the 2nd Primarch being a dour and humourless character. Given that this a brotherhood that included people like Perutabo and Lion El'Johnson, he must have been a real party-pooper for that to stand out as his defining personality trait.