Ever had a relationship with psychopath?

Yes but it was a friend. She was very manipulative and she was so charming that no one else believed that she was actually quite nasty if you did something she didn't like. Her main goal was to get to the top of the social and academic ladder and she is intelligent and very hardworking; it could have been mistaken for high school bitchiness apart from she was much sneakier and much better at what she did than others. She's also very attentive during conversations; she doesn't say much but she listens very closely and may bring up something you said in a seemingly innocent way to trip you up. She also shows satisfaction from hurting/ humiliating others; she feigns close friendships and tricks people into thinking they know her but when you ask, they realise they don't know that much about her. If she apologises for something, it's done in a very caring or dramatic way at close quarters but her eyes are completely blank and it seems almost forced. She hates people contradicting her and it's a struggle for her to not say something condescending in return.

How did this friendship affect you?

Ugh where do I begin.... It wasn't a friendship really; it was more one sided. Also, trust was something that was affected too; I am more careful making friends.

How old were you at that time? ..and when did you realise something was off with her?

Umm can I not say apart from that I was a teenager? Also, my parents told me; they didn't like how she acted and my dad has this weird thing for when people aren't quite as they seem and he got that all the time around her

I have that too, it's a gut-feeling you get. Do you admire some of her qualities?

Yeah also maybe it was his medical book that said she might be a sociopath.
She's hardworking and she's attractive I guess; the latter means that people are more likely to like and trust you anyway

Attractive female psychopaths tend to be the most dangerous kind, because they usually get what they want, especially seeing as most males are easy to seduce. How empathetic are you yourself, from 0 to 10.

Towards sociopaths? 1 or 2 towards the one I know anyway

In general, 0 being the zone of sociopaths, and 10 being extremely empathetic. It feels good to do bad things to bad people.

How did your relationship with her end?

6 or 7 maybe; I can be a little mean sometimes
It kind of fizzled out and we started ignoring each other; however, now we are back to being quite friendly (which is fake) around each other

Have you ever seen the mask slip? Or am I thinking of a different condition?

Define one please.

...

I've known a few online, on several fora. Without a doubt they had some seriously sociopathic characteristics. I have quite some stories about them.

I saw much of the same traits as PricklyPorcupine noted above. In addition, they are ruthless in going after anyone they suspect has their number and has sniffed them out, including recruiting toadies to do their dirty work. They surround themselves with subservient types as personal foot soldiers in their power play. They use their interactions to gather ammunition to know exactly what buttons to press. Masters at character assassination games and knowing exactly how to trip up, bait and trigger their targets. They love backstabbing, publicly humiliating their targets, and playing one side against another after turning on the charm.

Here's an example of a tactic they might employ: Suppose you have been talking to a woman who they know has had a stalker in her past, and thus is wary of stalkers. In the guise of a "heads-up", they would do their best to paint you as a stalker in her eyes, intentionally triggering her and pandering to her prejudices while feigning friendship with her, having gathered the requisite ammunition from her to use against you using her as proxy. Then she'd be goaded into backstabbing you as their toadie, while her penchant for gossip would also come in handy as a foot soldier in character assassination. Such common tricks are pretty much military-grade tactics, straight out of Machiavelli and Sun Tzu. The clever thing is that even if such games are known, that doesn't dissolve the suspicion.

A common disarming tactic I noticed is to ridicule another's behaviour (such as laughing at how another person tends to lash out at people who fail to boot-lick them), before strategically employing the exact same behaviour themselves towards you - then accusing you of doing what they are doing! (They might then defend the very person they ridiculed to you and turn them against you, after charming them.) The latter generally has the desired effect of putting one on the defensive, which they can easily spin as evidence of emotional instability as a discrediting tactic to further their character assassination games. Then those on their sides will deny they are capable of that and accuse you of slandering them for pointing out the facts. Then their allies will take their slander of you as fact. In effect they get others to project their own unsavoury characteristics onto those they target. They will usually cap it off by getting everyone to think you are playing the victim and thus attempting to manipulate them!

I've seen all the above and more in action on the Internet. It's not pretty. Then again, the sheer cunning of it all is mind-blowing. They basically know how to play everyone's weaknesses like a fiddle and amass an army of sycophantic followers, who are hugely impressed by their superficial charm, status, confidence, power and supposed strength. Which of course is immensely gratifying for them.

My experience of first love was with a psychopath, unfortunately.
Some of the things I noticed, was that they put on the charm offensive for a few months, and intermittently throughout the relationship (which was around three years long). The times they werent were awful. They did things like cheat on me uncountable times, and when I found out, they would tell me it was all my fault and that If I only payed more attention to them they wouldn't. OR they would deny it over and over again, tell me I was cheating, and that I was a horrible person. Then they'd eventually admit it. Other things they did was brag about all the girls they got with, and then ask me if I was worried about them cheating with a specific person. If I said yes, they'd tell me there was nothing to worry about, and that they "loved" me.. then I'd find out I had A LOT to worry about a little while later. They also pretended to be largely emotionally unstable, and would then get very violent and blame that. You might think that maybe that was the case, but he admitted afterwards that he just wanted to hurt me to get back at me, but for me to think that he couldn't help it so I wouldn't flip out. Every time I did flip out (get mad, not crazy) he would start crying and say he hadn't done anything wrong and that I was crazy and that I was abusive etc..
That all messed me up pretty badly, and I ended up at counseling. It ended a few months ago, and looking back I don't know why I stayed with him at all.. love I suppose. But it took a lot to realise that he didn't love me, and would probably never love anyone properly. It ended because he found a new target. I feel bad for her, but I'm glad I'm out of that now. Lesson learned