There's really four distinct Shadow Viper incidents.
1st: when they played a part in Empress Ayame's death.
Your neko is too young to have been involved in that.
2nd: The first Shadow Viper incident that targeted members of the Ketsurui clan within the Empress' Palace, Yuumi and Kotori in particular. It was the first time that it was implied that samurai killed fellow samurai - which at the time was inconsequential because the Empress' Palace was meant to be bombed in ways to frame the Independent World League for the incident. Had the ambush on Kotori not failed, it's likely the Shadow Vipers would have maintained their cover and remained unnoticed.
Probably the highest likelyhood of personnal involvement for the character in question if she needs first-hand account.
3rd: The second Shadow Viper incident was on Planet Kotori, involving individuals on the Samurai Squadron Y.S.S. Himeyuri. The traitors first attacked Kotori's retinue on the surface, almost claiming the life of Ketsurui Sora before they were eliminated. When Kotori's yatch escaped the system, the Himeyuri went in pursuit, but just as it was about to board the Yatch it sprung a trap that allowed those it pursued to cripple its FTL capabilities. The yatch hyperspaced away, leaving the Himeyuri stranded until Yui herself made it to the scene with her Star Army forces. Rather than be captured, the Himeyuri self-destructed.
Little likelyhood of personnal involvement, though it was a huge loss of face not only for the Ketsurui Samurai but also the Samurai Squadron. This was the first actually roleplayed encounter.
4th: The attempt on the Empress at the Christening Ceremony
Not really relevant to the creation of this character, though its impact is still being covered in thread 2.9 of my current plot.
I support Kim's recommendations. That's all good stuff.