For instance: a very quiet person, who does good deeds all the time and out of nowhere they turn out to be a completely different person. In animal terms, the hippo looks to be innocent, but is actually extremely poisonous and dangerous.
Wolf in sheep's clothing
- someone or something that seems to be good but is actually not good at all
My grandfather was a wolf in sheep’s clothing – he looked like a sweet old man, but he was really mean.
- Having no adverse effect; harmless.
Depending on your sentence, innocuous looking might work better.
Thanks to @AndyT: or seemingly innocuous.
Be overly coy or demure; be insincere. For example, She looked quite innocent, as though butter wouldn't melt in her mouth, but we knew better. Already a proverb in John Heywood's collection of 1546, this metaphoric expression alleges that one is literally so cool that butter inside the mouth would not melt.
Used in my experience of someone who is actively attempting to appear innocent whilst not being so. Often used sarcastically where the attempted deception is transparent, but also as a warning where the deceit has apparently been successful.
They are perhaps disingenuous.
disingenuous ADJECTIVE FORMAL
not really honest or sincere, and only pretending to be
You could also use snake in the grass
On a somewhat different tack than other answerers, what about the word Machiavellian?
Most dictionary definitions just say "someone adhering to the principles of Machiavelli" and I think that's because it's just difficult to define precisely what is considered "Machiavellian." On dictionary.com:
- characterized by subtle or unscrupulous cunning, deception, expediency, or dishonesty:
I would define it more as someone who is ruthlessly pragmatic and without conscience. Such people may appear guileless and helpful at first, until or unless they discover a way to use you or the situation for their own profit. The word is often used to describe psychopathy/sociopathy, because psychopaths so often are superficially charming, but only behave that way because it gets them what they want.
The adjective insidious is close to your intended meaning:
- intended to entrap or beguile: an insidious plan.
- stealthily treacherous or deceitful: an insidious enemy.
- operating or proceeding in an inconspicuous or seemingly harmless way but actually with grave effect: an insidious disease.
There is a a phrase from popular culture that fits this meaning, but it's not literary in origin, rather it comes from the low-brow realm of professional wrestling.
When a virtuous or good person suddenly and surprisingly turns into a villain, they are said to turn heel. Note that this is a different meaning of "turn heel" from the older phrase meaning to turn around, as in to turn heel and run.
This phrase originates in the world of professional wrestling, where wrestlers are generally divided into heroes, called "Faces" (from babyface) and "Heels" (from the early 20th century slang term for a disreputable person) being the villains. The sudden switch from face to heel is a way to generate drama and excitement.
Professional wrestlers turning heel may not be highbrow entertainment, but it's definitely big business, big enough to earn mention in Forbes magazine, for example.
The popularity of this entertainment has led to the terminology being used in other areas, such as politics and The same tropes also occur in daytime TV dramas (soap operas.) Writers will use this device to help stir interest in a story, but I can't find any specific phrases with this meaning from that genre.
Depending on the exact context, that person might be considered a subversive, which is typically defined as:
(n.) a subversive person.
synonyms: troublemaker, dissident, agitator, revolutionary, revolutionist, insurgent, insurrectionist, insurrectionary, renegade, rebel, mutineer, traitor "she was designated as a dangerous subversive"
As you can see, these synonyms all broadly cover the same meaning - one who is hiding amongst the crowd, pretending to be the same as the majority, while secretly working to undermine the status quo.
To directly answer your example, a subversive may do good deeds all the time as part of their disguise, but out of nowhere they turn out to be a completely different person when they see an opportunity to realise their hidden agenda.