There are a couple of places in our latest book 100 Character Histories where I used the term “pocket critical”. I’m not sure if this is a term I just made up or not, but a “pocket critical” is a character that has been designed so that they can get a critical when one is needed. They keep a critical in their pocket - get it? Most games that allow for criticals have some mechanic by which you can increase your chances, and these characters use/abuse those mechanics. So a character who is or holds a “pocket critical” is the one you go to when you absolutely have to have that critical hit.
Hope that explains it. and if you need to see how you can make that happen, check out 100 Character Histories (available at either RPG Now or Warehouse 23). Not only does it have 100 character backgrounds you can use, but each of them is named, so you can just use the names if you need. But we weren’t sure that people would understand our histories and our short hand, so we also included 123 character archetypes. That way you can far more quickly understand where we were going and what we intended.