A completist is the collector who won't rest until they have every single card from a given set, year, team, player, or category. Every. Single. One.
If a completist is working through 2021 Topps Chrome Baseball, they're hunting down every card on that checklist — every base card, every parallel, every auto. No exceptions. Where most collectors might ignore the commons, a completist needs them just as badly as the hits, because you can't complete a set with gaps in it.
It's a grind, but the satisfaction of checking off that last card? There's nothing like it in the hobby.