Desktop Army Collab Event will rerun sometime september.
Sylphy II is already available in the shop, but the event will provide chances to obtain her unique customization parts.

event pages from the initial run for what those are.
https://cosmicbreak-universal.com/en/special/240402_desktop_army
https://cosmicbreak-universal.com/en/info/topic/events/12967
https://cosmicbreak-universal.com/en/info/topic/events/12981