Even if you unlock Captain, the item won't show up in your logbook until you've collected it. But how?
Guide to Getting Defensive Microbots
What is it? And how do you get it? And what makes it different from other items in the logbook?
Defensive Microbots is a new passive (that comes as an item) on the character, Captain.
To unlock Captain, you just simply beat the game (not obliterate yourself) on any difficulty. When you get Captain and start up a game, you'll notice that you have the item right away.
Because of the way that it works, you technically never collect it. You just... Have it.
So even if you unlock everything, your logbook might look something like mine right now...
So how do I fill it out then?
The only way you can legitimately get it, is to kill your doppleganger (using the Artifact of Vengeance) and hope the item drops. When you kill your doppleganger, it guarantees a drop from the list of items that you have when it spawned in.
If you kill your doppleganger while you are playing as Captain with no other item collected, it guarantees that the Defensive Microbots drop.
Note: Make sure you kill it properly, and not push it off of a cliff. The item doesn't spawn if that happens.