General opposites:
* Apathetic: Lacking interest or enthusiasm
* Indifferent: Not caring one way or the other
* Unenthusiastic: Lacking in zeal or excitement
* Dispassionate: Free from emotion or excitement
* Detached: Emotionally uninvolved
* Cold: Emotionally distant and unfeeling
Opposites related to specific contexts:
* Calm: Opposite of passionate in the sense of being fiery or emotional
* Objective: Opposite of passionate in the sense of being subjective or biased
* Neutral: Opposite of passionate in the sense of being strongly for or against something
* Unmoved: Opposite of passionate in the sense of being affected or stirred
* Uninterested: Opposite of passionate in the sense of being curious or eager
Other related words:
* Lukewarm: Showing only slight interest or enthusiasm
* Bland: Lacking in flavor or excitement
* Dull: Lacking in interest or excitement
* Boring: Lacking in interest or excitement
The best word to use will depend on the specific situation. For example, if you're describing someone who is indifferent to politics, "apathetic" might be the best choice. If you're describing someone who is calm and collected in a stressful situation, "dispassionate" might be a better choice.