I think often the person has genuine motive but the means can appear fake & manipulative much more so than is common to other types (where manipulation is likely a conscious motive). The desire for harmony & consensus can cause a shift in their expressions that read as phony or manipulative to others, but they are unaware of it themselves.
A few comments from Jung (w/ some editing, for the sake of brevity):
Fe as a function
- [Fe] has freed itself as fully as possible from the subjective factor, and has, instead, become wholly subordinated to the influence of the object.
- May feel constrained, for instance, to use the predicate 'beautiful' or 'good', not...from my own subjective feeling, but because it is fitting and politic so to do
- ...in no way a simulation or a lie - it is merely an act of accommodation
- as soon as the object gains an exaggerated influence...[Fe] draws the personality too much into the object.... [and] the personal character of the feeling, which constitutes its principal charm, is lost.
- Feeling then....betrays a secret aim, or at least arouses the suspicion of it in an impartial observer. No longer does it make.... impression.... of genuine feeling; instead, one scents a pose or affectation, although the egocentric motive may be entirely unconscious [to the Fe person himself].
- Such overstressed, extraverted feeling certainly fulfills aesthetic expectations, but no longer does it speak to the heart
- Should this process go further.... feeling has entirely forfeited its original human warmth, it gives an impression of pose, inconstancy, unreliability, and in the worst cases, appears definitely hysterical.
The Fe Type
- Such an assimilation of subject to object then occurs as almost completely to engulf the subject of feeling. Feeling loses its personal character...
- the personality inevitably becomes dissipated ... Apparently, he is "this" one moment, and something completely different the next - apparently, I repeat, for in reality such a manifold personality is altogether impossible [the person doesn't necessarily change internally]
- The basis of the ego always remains identical with itself, and, therefore, appears definitely opposed to the changing states of feeling. Accordingly the observer senses the display of feeling not as a personal expression of the [Fe type], [not] as an alteration of his ego, a mood.... Corresponding with the degree of dissociation between the ego and the momentary state of feeling, signs of disunion with the self will become more or less evident
- This reveals itself...in extravagant demonstrations of feeling, in loud and obtrusive feeling predicates, which leave one... somewhat incredulous. They ring hollow; they are not convincing. On the contrary, they at once give one an inkling of a resistance that is being overcompensated, and one begins to wonder whether [the Fe type's] feeling-judgment might not just as well be entirely different. In fact, in a very short time it actually is different. Only a very slight alteration in the situation is needed to provoke forthwith an entirely contrary estimation of the same object.
- The result of such an experience is that the observer is unable to take either judgment at all seriously. [The observer] begins to reserve his own opinion. But since, with this type, it is a matter of the greatest moment to establish an intensive feeling rapport with his environment, redoubled efforts are now required to overcome this reserve
And Van Der Hoop (says same things in simpler language):
- Very soon find out the soft spots in the feelings of the people in their environment
- Seeks to affect others' feelings & create consensus, sometimes through very subtle means imperceptible to others
- Swift understanding of the attitudes of others with an immediate adjustment of their own reaction
- A vital need to find corresponding feelings in others
- They know how to get at the feelings of others
- More likely to be repressed than other types, condemning anything in themselves & others which might disturb harmony, something necessary to them
- Due to this repression, they may manage to conceal from themselves certain pronounced qualities, but to some extent these may be apparent to others, giving the impression of something artificial in their harmony, and something a bit unreal in their idealism.
- Such people are able to exert great influence in their environment, particularly if they find support for their feelings in followers and onlookers
actually probably not the devil maybe Lucifer please and kindly stay away 
