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What is your favorite obscure Retro Game?
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Mine is Lady Sia for GBA. It's just a platformer but I just love it played and completed more 20 times. Will probably speedrun it in future.
The game you're thinking of is Bangai-O Spirits: https://en.wikipedia.org/wiki/Bangai-O_Spirits. It was a DS sequel to Bangai-O on the Sega Dreamcast (another excellent, and imho the superior version). I had the DS version as well and it was a great game. If you ever get the chance (emu or native) I'd highly recommend playing the original on Dreamcast. Pure Arcade shmup goodness.