'des' vs. 'les'

[Jervon Voltage]

Hello,

 

Démétrius et Paula sont des amis.

 

First, is 'des' used as an 'article indéfini' in the sentence above?

If so, I can't figure out why 'des' should be used in such a case where we exactly know whom we're talking about (Démétrius et Paula)... To me it's 'les' which would fit there...

 

Thank you.

[Erwan]

Démétrius et Paula sont des amis  : It means "Démétrius and Paula are friends"

If you use les instead of des then a part of the phrase is missing. They become friends of someone.

Démétrius et Paula sont les amis de Martin.