You never truly truly know anyone. The persona they use for you, is different to their work persona away from you, different to their mates from school/college persona. You know the person they show you.
I feel.this argument has its flaws. I've been involved with my partner for a long while; in the course of that - cohabiting, going to his work functions, being out socially with his friends, hearing him talk to his friends on the phone, in recent years, hearing him all day while he works from home ...... It doesn't "compartment" out neatly like that at all.
Over the course of a long period of time, people find it hard to filter and tailor their personality for their cohabiting partner in that way. Especially not if alcohol is ever involved, which it usually is.
You'd have to be very obtuse not to pick up on their full personality, particularly over a long period of time, if you're around them constantly.
In my experience, lots of women do pick up on traits in their partner, but ignore/dismiss/minimise/make excuses for/stick their head in the sand etc about him. Because their want for a partner and family supercedes it (and they believe all men to be sleazy/predatory/dick led/base etc.anyway, so they think they won't get different or better ...and they've got their window to grab a partner and maybe marry and have kids, while everyone else is doing it and they don't want left out or left behind).
If older when they discover things, they don't want to face old age alone.
Other women don't really pick up on traits, because they project how they think & feel .... and because they're not predatory, base, sex oriented etc. etc. they can't imagine what it is like to be so. They are extremely naive in a way.