Reticent

[Rihanna]

Inclined to keep silent; reserved; uncommunicative.

[Julian]

A [demeanor] by which the individual does not [readily] display their thoughts or feelings. This individual may make an excellent [undercover] government operative or ninja.

[Hina Muqadas]

That character in [RPGs] who is always dark, [moody], and [barely] says anything.

[Dianne]

unwilling to speak about your thoughts or feelings:

[Kit]

Not revealing ones thoughts or feelings readily.