Agent Twilight, aka Loid Forger, is the most achieved spy in the nation of Westalis. To get to this rank, Loid has dedicated his life to studying people and figuring out exactly what makes them tick. He knows precisely what people expect to see day to day, and uses this to his advantage to hide his real identity throughout Spy x Family.
However, this is exactly why Loid's fake wife, Yor Forger's, assassin identity will always escape his suspicion. By day, Yor cares for their adopted daughter Anya and goes to work as a clerk at city hall, but by night she slips into her alias as the deadly Thorn Princess.
Yor Actions Are Unexpected In Spy x Family
Yor is personified by her warm, ditzy and naive appearance in Spy x Family, which makes a perfect cover for her secret identity. While in her inner monologue she comes across as ruthless and brutal, to anyone who can't read minds like Anya, Yor is just a little strange and aloof. She's so incredibly strong that it's easy to write off her personality and obscene abilities as her being weird, and not expertly trained in combat and assassination.
Yor's knowledge of combat is so abnormal that it's easy to question whether she was actually trained or was just born knowing how to land a killer blow. Yor also knew how to incapacitate a cow, something unlikely to have been taught in hand-to-hand combat. This is just another one of her quirky and downright weird traits that make the Thorn Princess so difficult to suspect.
Yor Is The Ideal Assassin Thanks To Her Quirky Personality.
This plays into Yor's assassin role perfectly as no one -- not her secret police brother, spy husband or her jealous co-worker -- would ever suspect that she murders people for an organization called The Garden under the alias Thorn Princess. They may suspect she's trained in self-defense or is just an odd person, but this removes her entirely from the subtleties of a spy war.
Everyone is aware of the secret police, as well as the spies working for the government. Twilight is basically a household name; but not once has The Garden been mentioned, let alone Thorn Princess. Loid probably assumes from intelligence reports that the people Yor kills are murdered by the secret police. In the end, how could Loid suspect an organization he isn't even aware exists, much less his own ingenuous wife?
Loid Forger Was a Victim of His Own Trick.
Spy x Family's Loid Forger is definitely neither an idiot nor easy to fool. However, he has fallen victim to what he himself uses to exploit his targets. When it comes to Yor, he is seeing what she expects him to see, not what's actually there. Her perfectly crafted empty-headed naivety disguises the Thorn Princess because Loid would simply never expect her to be a stone-cold killer based on what he's seen.
He knows she's suspiciously strong, but he also knows that his wife is a complete weirdo, so really it fits with her character. Yor's personality is almost suspiciously crafted, as if she trained in how to fool people into believing she's an innocent civilian with a strange plethora of combat knowledge. For all Loid knows, Yor went to self-defense classes to protect herself from general danger -- which is entirely feasible for a woman to do.
Loid knows all about how normal women act, and how a family dynamic looks. He's a spy and uses that ability to shape-shift into whatever he believes the subject expects to see. What he didn't expect, however, was for his wife to do the exact same thing.