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

I am trying to find a word/phrase that describes someone who performs a "reality check" every time people suffer from misconceptions or over-exuberance. He/she is the "voice of caution" that prevents "fools from rushing in." A sage with a sobering influence.

What I'm looking for could be an eponymous adjective such as panglossian (though obviously panglossian is nearly an antonym for what I actually want). It could be a noun - such as Cassandra - if such a character personified the above traits. Any idiomatic expression or slang that might also fit the above criteria would also help.

The closest I have gotten so far: pessimist, skeptic, realist.

I want to use the word/phrase in a sentence such as: "I hate to be a realist/prophet of doom/Cassandra/killjoy, but there is no way we can hit these goals in just a week!"

I think Devil's Advocate fits in certain scenarios. A Devil's Advocate is

a person who advocates an opposing or unpopular cause for the sake of argument or to expose it to a thorough examination.

In your case, the devil's advocate might point out possible problems with the plans of the group, and if those points prevent the group from doing what they were planning, that person could be seen as a kill-joy.

Example:

Person 1: "We should go cliff jumping!"

Person 2: "That sounds great!"

Devil's Advocate: "Just to play Devil's Advocate, we are predicted to have thunderstorms later, so cliff jumping may be a bad idea. Also, didn't you break your arm doing that last year?"

Person 1: "I guess you're right, why do you always have to ruin our fun ideas?"

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.

Party-pooper?

party pooper

ˈpɑːtipuːpə(r)/
noun informal

a person who throws gloom over social enjoyment.
"I hate to be a party pooper, but I've got to catch the last train"

As the definition says, they are sort of the voice of reason. Are you going to do anything irrational as fun? Someone might burst that bubble, and turn into a Party pooper!

Although neither an adjective nor a noun, “[Sorry/hate to] rain on your/this parade" is an idiomatic phrase that would work in your example sentence:

"I hate/am sorry to rain on your/this parade, but there is no way we can hit these goals in just a week!"
(example usage from ‘Please Don’t Break An Angel's Heart’ by Faith Ford, via ‘Google Books’)

rain on someone's parade and rain on someone or something
Fig. to spoil something for someone.
I hate to rain on your parade, but your plans are all wrong.
She really rained on our plans.

(definition/meaning from ‘McGraw-Hill Dictionary of American Idioms and Phrasal Verbs’ via ‘The Free Dictionary by Farlex’)