To get Siren, speak to Klavia near Yach's shop. After speaking with her multiple times about her singing, she starts to practice more so her voice can improve. This will allow you to make a pact with her friend, Siren.
Siren; Daughter of Sound, is Nell's Blades Master, obtained after defeating her in a Mana Boss Battle. Nell's blade in this form is a bell, resembling the sound affinity. This form trademark ability is "Trick or Treat", which can turn 1 non-champion, immune magic, or boss monster into items that might it drop when defeat it. In short, it guarantees that the monster always give item, but at the cost that turned monsters won't give bounty when killed.
When summoned by Iris into battle, Siren will blow her trumpet, dealing magic damage to all enemies.