Something less roguish than "imp" or "wag." I'm thinking about a person who is perhaps a bit sullen or in need of some levity. The word or phrase would be better if it expressed how this person feels rather than labelling the type of person they are. Maybe a type of sentiment.
not friendly or forthcoming; cool and distant.
conspicuously uninvolved and uninterested, typically through distaste.
What about diffident or one of its synonyms?
modest or shy because of a lack of self-confidence. a diffident youth
late Middle English (in the sense ‘lacking confidence or trust in someone or something’): from Latin diffident- ‘failing in trust’, from the verb diffidere, from dis- (expressing reversal) + fidere ‘to trust’.
synonyms: shy, bashful, modest, self-effacing, unassuming, unpresuming, humble, meek, unconfident, unassertive, timid, timorous, shrinking, reserved, withdrawn, introverted, inhibited; insecure, self-doubting, doubtful, wary, unsure, apprehensive, uncertain, hesitant, nervous, reluctant, fearful; self-conscious, ill at ease, ashamed, abashed, embarrassed, shamefaced, sheepish; mim; (informal) mousy — From Google Dictionary
socially detached and unresponsive : exhibiting withdrawal : introverted
Examples of withdrawn in a sentence:
He became more withdrawn after his brother's death.
She was withdrawn as a child but is now more outgoing.
My word shall be unforthcoming. It is carried by 4/5 dictionaries:
Collins English Dictionary
not inclined to talk about something ⇒ she was unforthcoming about her past
Example sentences containing 'unforthcoming'
- He sat at a small table, earphones and headset on the surface before him, an unforthcoming expression on his face.
Smith, Evelyn E MISS MELVILLE REGRETS
- He was an affable chap, though unforthcoming on personal matters.
Fraser, Anthea THE GOSPEL MAKERS
There is an idiom common in AmE, in a funk (or a blue funk)
(chiefly North American) A state of depression: I sat absorbed in my own blue funk
Oxford Dictionaries Online
It tends to be used for a transitory condition rather than a chronic depression.
This person is down in the dumps. It is usually a temporary condition, for example:
I don't want to think about anything serious. It's been raining for three straight days, my cat is sick, and I am down in the dumps.
down in the dumps, From The Free Dictionary:
unhappy. She's down in the dumps because all her friends are out of town.......
Also, down in the mouth. Discouraged, depressed, or sad, as in She's been down in the dumps ever since she lost the match, or What's wrong with him? He's so down in the mouth about everything. The noun dumps has been used for "a state of depression" since the early 1500s, and down in the mouth, alluding to the downturned corners of the mouth as a sign of misery, dates from the mid-1600s.