This has been updated to match my "Signature Weakness Project." I have done my best to make sure that the original content isn't altered too much, out of respect for any comments.

Another weakness aimed square at the investigator's special ability. Looking at the two elements, the effect and the discard condition, we get:

The effect: You can't add assets to the play area. Dexter likes putting assets into play, it's kind of his jam, and this card stops that hard (see the note at the end about interpretation of the text). This is another very situational weakness. While you are never happy to see your demonic "friend" show up, sometimes it absolutely wrecks you, while other times you can limp to the end of the scenario without too many regrets. In some ways, it the exact opposite of Searching for Izzie or Cover Up, in that, if you have to draw it, the last turn or two is when you want to do do so. As usual with the to remove weaknesses, this has much greater impact in solo than multiplayer.

The discard condition: Spend two actions.

Taken together, this is a below average signature weakness, maybe average in solo.

Box vs book While this weakness is probably less annoying overall than Occult Scraps, the "box" signature weakness, neither is too terrible, making Dexter one of the investigators who should think seriously about including both sets of signature cards in one deck.

Note: The card text is a bit puzzling, since it seems obvious that the meaning is that Dexter cannot add assets to his play area. However, the Rules Reference makes clear that "play" and "put into play" are completely separate actions, suggesting that Dexter is only forbidden Anna Kaslow and Sleight of Hand, which would require creating an entirely new tier of "This is barely a weakness." When asked about it, MJ Newman responded "The intent is that you cannot do either—play assets or put them into play—the idea being that playing something does in fact also put it into play. However, as you rightly point out, that’s actually incorrect as far as the rules are concerned, so we’ll be updating the rules to reflect this in a later edition of the FAQ." So, Yaztaroth prevents any assets for Dexter.