expression-requestsidiom-requestssingle-word-requests
The hidden purpose can be selfish or not. He or she is involved in something to ensure their purpose is fulfilled.
Example : She joined our group to help us. She has no _____.
He will only join us in this quest if he has ____.
Or am I asking too much and should settle for hidden purpose ?
I'd call that person a
contrarian: a person who takes a contrary position or attitude
I would go with disillusionment.
A feeling of disappointment, akin to depression, arising from the realization that something is not what it was expected or believed to be, possibly accompanied by philosophical angst from having one's beliefs challenged.
An example from vocabulary.com:
Disillusionment is when the hard truth of reality makes you lose faith in your dreams and ideals. Like you might have thought your internship at the movie studio would be super glamorous, but you quickly suffered some disillusionment when you realized it involved 14-hour days and lots of coffee runs.
A deep distinction between disillusionment and disappointment from the book "The Poetics of Disappointment: Wordsworth to Ashbery" By Quinney:
Best Answer
Consider using hidden agenda:
[American Heritage Dictionary of the English Language, Fifth Edition]
As @Phil Sweet commented, you could also consider using "personal agenda" or just "agenda". I've just found a related question, Meaning of “have an agenda”.