Echidna Iisaki is the 16th in Shadow-Mirror scientist Lemon Browning's W-series of artificial humans. Much like other members of the W-series, she displays a cold, emotionless demeanor and follows orders blindly, prompting Shadow-Mirror ace Axel Almer to label her a "Puppet" and express his distaste for her.
Echidna was sent to infiltrate Archibald Grims' squad in the Neo Divine Crusaders, under the guise of bringing him new mechs to use. In fact, her objective was to disable the EFA's Izu Base, as well as to locate fellow W-series member Lamia Loveless, who had been separated from the rest of the Shadow-Mirror and wound up infiltrating the ATX Team. Upon finding Lamia, whom Echidna refers to by her model number, W17, she relays new orders to keep watch on the Hiryuu Custom and Hagane crews, and be on the lookout for Helios Olympus, whom the Shadow Mirror wished to use to complete the System-XN and fuel their ambition to create a multi-dimensional war without end. She also supplies Lamia with a new transceiver, as the one on Lamia's Angelg mech had been damaged.
Echidna spent the remainder of the war blindly following orders and participating in missions as ordered by Lemon and Vindel. She was one of the first to observe that Lamia had begun to develop a sense of self, believing it to mean that the fellow W-series had some sort of defect. During the EFA and Einst assault on Neviim, Echidna sacrificed herself to save Axel from an attack by Kyosuke Nanbu. While Echidna stated that she was simply following orders from Lemon not to allow Axel to die, Axel pointed out that those orders did not say anything about throwing her own life away to do so, leading towards the possibility that, like Lamia, Echidna was beginning to develop her own sense of self at the time of her demise.