A “completionist” is, for our purposes, someone whose interest in “finishing” something or being “thorough” extends beyond the realm of normal enjoyment, and particularly one for whom the assiduous completion of an activity is an intrinsic good apart or even contrary to the normally enjoyable aspects of the activity. Andrew’s obsessive FF6 levelling is a textbook case of completionism, for example. Andrew and Nick are also Cusslerian completists whose thirst is as yet unassuaged. Completionism probably also had something to do with their finishing Eternal Darkness three times (to obtain the special ending) and something to do with Andrew’s repeated attempts at beating the Michael section of the game (where you actually get decent guns) using only a fire ax.
The term is obviously related, though not directly, to “completist,” which appeared in 1955, when it was used in the NY Times Book Review as follows: “Perhaps the one point of interest here, even for the Holmesian completist, is the effort in the introduction to identify Watson as Doyle’s secretary, Maj. Alfred Wood.”
Both terms are clearly closely aligned to Nerd Culture of various kinds.