Can the "Cas9"(involved on the Crispr-cas9 mechanism) be considered as a restriction enzyme?
Cas9 is an endo-deoxyribonuclease (UniProt-Q99ZW2). It also has a 3'-5' exonuclease activity by which it trims the DNA a little bit, from the cut site (but not too much).

Although, Cas9 is an endonuclease and is evolved as a mechanism of immunity against viruses (like restriction enzymes), they are not considered restriction enzymes. One of the most important differences is that the restriction enzymes do not need an RNA or a DNA as a tether to identify the target sites in the DNA; their DNA binding domains can recognize the restriction sites by themselves. 

Cas9 cannot just cleave any DNA complementary to the guide RNA. It needs PAM (protospacer adjacent motifs) sequences adjacent to the target site to cleave that latter.

For details check out the wikipedia page on CRISPR or read a review.