Horrible Judge of Character

Real Life manipulators appear convincing and interested in your own good, and soothe one in by being nice, kind, getting your sympathy and if that doesn't suffice for them to get everything they want, they begin pushing buttons as they lie whenever they can get away with it. All of that is so subtle that it usually escapes the people who it is happening to.

But in fiction, everyone else, especially the audience, can and will immediately identify the manipulator as evil, but the manipulated is simply a Horrible Judge of Character.

When only the protagonists see through the manipulator and everyone else holds him who is quite often also transparently mean, abusive and treacherous to everyone (who doesn't have the authority to punish her for it).

When the plot requires for this, Innocence and helplessness may attract guardians and friends, but will also make them vulnerable targets to get romantically involved.

On the extreme end, sometimes the heroes will also be intensely loyal to their friends, so they'll ignore all evidence that the Manipulator means them harm. When true friends try to point them to suspicious behavior or even show outright damning evidence, they will get a pouty "You're just jealous of our friendship!" and be blown off.

It usually takes a point blank Evil Monologue from their "friend" over the True Companion's corpse to even faze them into considering the possibility they might not be as hug-tastic a friend as they thought.

If it doesn't break them, then they'll just turn right back around and follow their "friend" around, say he forgives them for killing off thousands and betraying him completely and insist the Power of Trust and Friendship will redeem them.

Hero with Bad Publicity is the reverse of this where a character who is considered evil is actually good. If it's romantic, then admit it.

Related is Pacifism Backfire, where someone chooses not to act violent against someone else; one reason is that he still believes that someone is still good.

If the work expects the audience to not figure out that an Obviously Evil character will turn out to be the villain, its up to them.

Examples

 * The Sultan from Disney's Aladdin
 * Snow White completely trusts a creepy hag and thinks that she's just a nice "poor old lady", which shows just how tragically good and pure she is even her animal friends who immediately realize something is up can't convince her not to help.
 * Quasimodo due to being raised by Frollo for his entire life.
 * The Lion King: Both Mufasa and Simba seem to trust Scar. In Mufasa's case, it gets him killed.
 * The live action How the Grinch Stole Christmas embellishes Cindy Lou's encounter with the Santa-disguised Grinch as he's stealing her home's Christmas trappings. She tells him that the Grinch isn't a bad person, just lonely. The Grinch remarks to himself that she's a "nice kid", but a "baaad judge of character". Then, he later have a change of heart, turns out that she was right after all and it's actually he who falls into this trope.