(in Outsiders #16, Mar 1995)
The 3rd Syonide was a woman named Nikki. She was part of Lady Eve's new incarnation of Strike Force Kobra. Her teammates, all female, were Fauna Faust, Dervish, Spectra, and Windfall. (though later Windfall switched sides when she found out Spectra wasn't really Halo as she'd claimed to be, but Violet Harper instead). She and Fauna were girlfriends and in love.
She and her teammates ambushed the Outsiders and Nikki took down and captured Eradicator and Sebastian Faust. When the Outsiders woke in Lady Eve's base, there was another battle. During the fight with the Outsiders, Nikki told Looker that it was a shame they were enemies, because they could have made quite a threesome (unknown whether she was serious or not). While she was battling Looker though, Eradicator sneaked up behind her and seemingly murders her, choking her to death with her own whip.
Fauna was horrified and fled, going to her father, Felix Faust, hoping he would help her get revenge.
Weapons of Revelation
(in Action Comics #817, Sept 2004)
Syonide makes a cameo appearance in this issue of Action Comics as one of the many villains trying to take advantage of Superman's weakened state to kill him. It's unknown weather this was supposed to be Nikki, the 2nd Syonide, or a 4th Syonide entirely. She was wearing the 2nd Syonide's battle gear, but had shorter hair. Given that Nikki is later confirmed to be alive in Firestorm #17 the next year, it seems likely (but far from certain) that it's her in this appearance.
An Unrighteous World
(in Firestorm #17, Nov 2005)
Nikki inexplicably reappears in a later issue of Firestorm, very much not dead. Given Fauna's parentage, it's not unthinkable at all that she was able to resurrect her at some point with magic. Either that, or Eradicator didn't really murder her after all (though that seems unlikely given Fauna's storyline in subsequent issues of Outsiders). Regardless, it's never explained.
In this appearance, she's part of Queen Bee's team, along with Black Adam, Parasite, and the Royal Flush Gang. She battles Firestorm and Gehenna, but the two end up escaping in the end.
Nikki doesn't die this time, but never makes another appearance.
The second Syonide was recruited by the criminal underworld crime lord Whale soon after the original Syonide's death. She however was indeed the cold blooded killer which Tobias had wanted and assigned her as his personal bodyguard. Syonide was outfitted with a whip capable of discharging varying amounts of electrical voltage; a pair of guns that fired either poison or stun capsules, as well as conventional bullets.
The Whale then sent Syonide to track down the whereabouts of Violet Harper. Syonide tracked down Halo who had taken possession of Violets body and was at the Harpers' home trying to discover her true identity. In the ensuing battle with the Outsiders Syonide was shot and killed by Sam Harper who she in turn shot and killed.
The first Syonide, a man named Andy, was a hired hit-man for the underworld crime lord Tobias Whale, and fought the Metropolis based hero Black Lightning on occasion. However this was until he committed suicide due to guilt over the countless lives he had killed on behalf of the 100.