It seems Calypso is also a LI but a solo LI route outside of the Harem route
That's the thing though, she can't be a part of Orion's harem because it's the other way around: Orion and the girls are members of HER harem.
In all seriousness though where'd you heard the solo route part? Cari said she's romanceable/a love interest but she's different compared to the others. I only took that as her having fewer scenes to the point that she doesn't need a dedicated LI page and that her progression is more tied to the main story (so no dedicated activities or dates with her that feel separate from the main story), and not that her route would only blossom in a solo path (which, if my personal theory for her final fate ends up being true, will be a bittersweet fate for Orion in a sense).
does that mean there is a possibility for calypso to leave eternum and go to the real world ? (although the real world would stop being just another server, but anyway).
There's some who theorize (me included) that her having the ability to jump to any world and the various hints that suggest that Eternum is not actually a virtually constructed MMO are foreshadowing that she will eventually be able to cross over into the real at some point in the story. What triggers it and why she would even make the jump is anyone's guess, but IMO said event would make for a legendary update ender/cliffhanger*.
*And yes, this is reference to what Cari posted sometime on Discord and is a possible event that could happen that IMO counts as such...
Not that I'm saying that THAT'S what the cliffhanger's gonna be but it's at the top of my list on what could potentially happen by the end of 0.8 (others on the list include "Orion dying and finding himself respawned in Eternum," "Praetorians mark party for excommunication and update ends with them blasting Orion and cutting to black (and then credits roll) before said blast hits," and "There actually is no cliffhanger this time. Cari's just fucking trolling everyone.
")