Sirius ∑

Ryuusei no Rockman 3 : Red Joker
Sirius ∑

I used a cheat code to battle this guy in the emulator. But after inputing the code for ∑ bosses at the title screen, you can find him legitimately (randomly) at the Orbital Base.

== From GameFAQs ==
http://www.gamefaqs.com/portable/ds/code/946070.html

Make Sigma bosses appear randomly

At the title screen hold L and press these gold stars in order: S Comp Star, G Comp Star, S Comp Star, M Comp Star, SS Star, SS Star, and Black Ace Star. You should hear a confirmation if done correctly.