A Completionist Name is the name given to certain friends of Jirard Khalil, ranging from serious to whimsical.
Jirard's "Completionist Name" is the original; The Completionist. His name stems from the fact of him completing games week after week, and has earned him said title.
Greg's Completionist name comes from his general badness in gaming; The Mediocre-ist. This name first appeared at the end of the Super Meat Boy episode. In honor of Jirard's death at the end of the episode, he dubbed himself the Mediocre-ist.
Alex's Completionist Name came between the DLC Episodes 7 and 8; The Proof-ist. Originally, Alex didn't have a Completionist Name, and in his debut episode, DLC Episode 6, he was simply known as Alex. In the next DLC Episode, he was known as The Nameless-ist, serving as a make-shift Completionist Name. By the time of the next episode, he had obtained his actual Completionist Name; The Proofist. It was never explained how he obtained this name, however.