I just pulled Kari Valkyrie Siren from a rare soul. I was under the impression that she was an event exclusive character and was not available from other souls. To quote @Sparton from that event -

"Additionally, this character is not available in any other souls, so if you miss your chance to recruit Kari, it may be a very long time until you get another chance!"

About 10 weeks in fact....

I still can hope of getting the Gunner Chopper and Sentinel Mayan Priestess that I missed out on then in the coming weeks?
Although I think its a bug with the dupe system so since I dont have these 2 its far less likely I'll get them in the future.