Sailor Iron Mouse was the first animate the Sailor Scouts met when Sailor Galaxia and her servants came to Earth. She was quite a shock to the others, who didn't believe there were sailors anywhere else but their solar system ( a shock doubled when the Sailor Starlights appeared shortly after Mouse). Iron Mouse, as a human, went by the name Chuuko Nezu(chuu meaning to squeak,-ko meaning child, and nezu short for nezumi, meaning rat/mouse), and posed as a TV director. Often she was dressed in pinstripes(really cute!) with fedora, like a 1920's gangster, and when she trasformed it was nothing fancy, she just ripped off the pin-stripe suit with a "Chuu!" and poof, there was Sailor Iron Mouse. Mouse did figure out that the starlights where Sailor Senshi, even tricked them into transforming. She almost got the Sailor Starfighter's starseed, but Sailor Galaxia became impatient with Mouse's failures un the untimely way boss villians do, took away her bracelets, and killed Mouse :(.
In the Manga, Iron Mouse had brief appearance. She sensed the Starlights at their concert, and attacked, blasting the scouts and audience with her Galatica Crunch. She had the starseed of Sailor Chuu. The Scouts could not defeat her, since her bracelets absorbed their attacks, but the Star Sensitive Inferno of the Sailor Starhealer killed her off.