Sailor Aluminum Siren is the 2nd animate to appear. She is my personal favourite.  She's often associated with Mermaids (which is actually because in the manga, her starseed once belonged to Sailor Mermaid).  Her attack is "Galactica Tsunami" (and it's actually quite spiffy looking!).   She had this habit of eating a lot when things went wrong.

She was Lead Crow's student.  They had a very interesting friend/rival student/teacher relationship.  In Eps. 188 when Siren was killed by Galaxia, Lead Crow begged for Siren's life. However, she was a great Metal Knight, and the only one to discover Sailor Moon's identity.