phrase requests - Is there a word for someone who is a killjoy yet also "The voice of caution?"

While not suggesting that either of these terms has entered into usage as stand ins for their characteristics, if you want fanciful you might consider Mr. Spock or Lieutenant Data, both characters in the Star Trek series (original and Next Generation respectively).

Both of them are characterized by decision-making informed by facts and precise logic, and a rejection of emotional influence on the process.

Supplement: More conventionally, there is spoilsport

(informal) a person who spoils the pleasure of other people by his or her actions or attitudes

Collins

Similarly wet blanket

(informal) A person who spoils other people’s fun by failing to join in with or by disapproving of their activities.

Oxford Dictionaries Online

Finally, there is worrywart

(North American informal)
A person who tends to dwell unduly on difficulty or troubles.

Oxford Dictionaries Online

This last term suggests overbearing and perhaps unnecessary hesitancy.