For those who have not completed games in the Xenoblade Chronicles series, this page contains spoilers regarding the plot. Discretion is advised.
After finishing the game, installing Patch (Ver.1.3.0), and starting a New Game Plus, Siren will take part in a Level IV Blade Special for Pneuma. It will fire a gigantic beam towards Pneuma's desired location, inflicting large amounts of damage.
Through a flashback, it is seen that Siren simply refers to an Artifice model that was used to defend the Beanstalk; the model's full name is Echelon Siren. According to the Siren Model kit released by Monolith Soft, Siren and the other artifices were not created by humans, but rather by the Trinity Processor itself. Siren does not have any internal engine and instead uses a device known as a "Slave Generator" to receive energy from the Conduit. In the year 20XX, the Saviorite rebels easily overpower the Echelon Sirens, and the Beanstalk director orders the activation of Aion, though Klaus soon runs his experiment that would ultimately create Alrest. Though there are multiple Siren's in space above Alrest, Mythra has control over one specific model. Malos too had control over his own Siren matching his dark color scheme.
In the Aegis War, Malos uses his Siren's power to destroy Titans like Coeia. However, he grows tired of letting Siren destroy the Titans and aims to destroy Torna by reawakening the Titan and attacking its core. When she was first awakened, Mythra used Siren's power freely and without restraint, as seen in the battles against Jin and Lora and against Brighid. Milton, Addam, and the others criticize her frequent use of the Artifice.
In the final encounter with Malos at The Soaring Rostrum, Malos and Mythra both summon their respective Sirens to do battle. After a brief fight, the two Aegises pilot their respective Sirens themselves, much to Addam's objection. Mythra releases a powerful attack resembling the Hilbert Effect which sends both Malos' Siren with Malos on board and the Tornan Titan sinking into the Cloud Sea. Malos' Siren was destroyed in the battle. Upon seeing the destruction, Mythra turns into Pyra and vows to never use Siren's power again.
When Mythra is first re-awakened at the Olethro Playhouse, she uses Siren's power to send blasts of light down on Akhos and Malos. This power eventually reverts their Blades, Obrona and Sever, back into their Core Crystals. Mythra tells Rex that she cannot use Siren's power indoors, as it would result in the structure collapsing. She uses Siren's power again at Bulge Harbor to revert Mikhail's and Patroka's Blades to their Core Crystals as well.
When Jin unleashes his true form at Genbu Crown, Rex and Mythra attempt to use Siren's power to try and stop him; Jin, having control of elementary particles, is able to stop Siren's particle cannon and overpowers the two. As Jin is about to kill Rex, Pyra uses her willpower to summon Siren's targeting ray and threatens to destroy herself. Jin backs off, and Pyra releases control of Siren and hands herself over to Jin.
After Pyra and Mythra become Pneuma at the Gotrock Oracle Ruins, Malos summons Ophion to attack the party. Pneuma then summons Siren, and the two Artifices engage in a battle that destroys the ruins and sends the party into the Land of Morytha. Siren wins the battle, and Ophion breaks free of Malos' command and stopped guarding the World Tree.
Upon reaching the exterior of the World Tree, Amalthus, using Fan la Norne's Core Crystal that he implanted in himself, attempts to control Mythra and summon Siren to attack Torna, though Mythra resists. With Poppi's help, Rex and Mythra get close to Indol, and Mythra uses Siren to destroy Indol's amplification towers and prevent Amalthus from controlling Blades.
Various Siren models along side various Gargoyle models are seen attacking Alrest at the end of the game under Malos' command. However, by directing a blast of Mythra's Siren into Pneuma's Blade, Rex is able to defeat Malos, and the other Sirens halt their attack on Alrest. After the Conduit's disappearance, they begin to fall down to Alrest.
In addition to the Pyra figure previously announcement by Nintendo, Monolith Soft has announced the Siren and Mythra figures made into a 1/7 scale respectively by Kotobukiya and Good Smile Company.