The game was kind of intimidating at first with the scary music, theme, etc. but it's actually a pretty fun bullet hell. I liked it.
I tried on Linux, got a missing library error:
./rex_operari_demo62: error while loading shared libraries: libGLEW.so.2.2: cannot open shared object file: No such file or directory
So I played the windows version. Outside of the menu issues, it worked perfectly until the end. I noticed, however, that when you start the program it spawns a terminal window. You might already know this, but you can hide the terminal by using the "-Wl,-subsystem,windows" flag in gcc, or an equivalent of it.
It seems "load profile" function is broken. When I started the game, I tried to set the new profile's name as "Lily." It didn't work; I went back to main menu, tried to load profile, saw that there was already a "Lily" profile with a score of 9999; went back again, made a new profile with another name, then went back and selected "load profile" again; the game immediately crashed. I restarted the game, and it continued crashing when I selected "load profile." I reinstalled the game, went through the same motions, and it still crashes in the same way.
I played the easy mode. The stages are pretty short; I'm guessing you're going for a more boss-centric design, which is fine. The patterns are well made and nice to look at, but some of them are kind of unfair. In some patterns it's possible for you get hit by a bullet that went off the lower edge of the screen; as far as I know, that's pretty unusual in shooters, since it should be "dead" at that point. I also had difficulties with the patterns where the bullets get sucked in but I ascribe that to me not using the portal (I didn't know how to use it for most of the game).
At the end, I got to the fourth stage (Parabola V), but when I got to the boss my shots didn't deal any damage to him. I assumed it was timed, dodged his first pattern for a while (100 seconds actually), but it didn't end, so I'm not sure what I had to do to get further. It might be a bug.