Neutral:
* Private
* Discreet
* Confidentially
* Close-mouthed
* Reserved
* Withholding
* Reticent
Negative:
* Clandestine
* Covert
* Underhanded
* Furtive
* Sneaky
* Devious
* Evasive
* Shifty
* Suspicious
* Hidden
Positive:
* Private (can be positive if referring to personal matters)
* Discreet (can be positive if referring to being tactful)
* Confidential (can be positive if referring to sensitive information)
The best synonym for "secretive" will depend on the specific context and the intended meaning.