Kouyoku no Soleil was the first Soleil game I ever played... and from a purely action perspective, it was fun... but it lacked something from my perspective because so much of the game is steeped in pre-existing knowledge from the other games (literally, this game pulls stuff in from Shirogane, Gouen, Soukyuu, and even Primary). 

Basically, this story starts with the Norns fighting one another over what to do about the original Norse Mythology world, which hasn't moved on from the moment the fate they predicted came to a close.  Touya, the protagonist of the story, meets the Norn of the Past, Urd and ends up getting dragged into her affairs as she tries to find the Valkyrie who was supposed to spark the new world into existence.  In the process, he bonds with the clutzy newbie valkyrie Rinasoel, ends up partnering with the magical girl (from Primary's world) Palfina, is repeatedly plagued by attempted sexual assaults by the firebird youkai from the Ayakashi realm named Yatohime, and ends up in a rather odd friendship with the girl beloved of the Lovecraftian Old Ones, Celaeno.  

This really... varied cast comes together because the world he lives in, the 'central' world, is a nexus of dozens of worlds, which in turn have connections to yet other worlds (though distant), thus leading to a rather chaotic situation.  Making it worse is the giant immaterial sword plunged into the central world, which stands silently in the background for much of the story.

To be straight about it, Touya is, like most of the protagonists in this series, a serious philanderer.  He falls in love and has sex with all the heroines I mentioned above, and he is rather passionate...  unfortunately, he is also rather incapable of the fidelity some of the more jealous ones (such as Rina and Palfina) desire.  Touya is devoted to protecting Kukuri, the hikikomori girl who is also his landlord and his boss for his part-time job at the convenience story.  His soft side is most often shown with her, in an almost fatherly manner.  He is strong and violent but highly protective of those he cares about and pretty compassionate, though he does have an immense potential for destruction.  His right arm can transform into a beast-like claw, which he can channel power through to sever anything, whether material or immaterial.

This VN is a kinetic novel  - save that it has bad endings - for the most part, with only one storyline, and that is probably its biggest saving grace, because it didn't share out the story between a bunch of routes that probably would have not been nearly as satisfying as it was.  Upon my second playthrough, I honestly felt that the game deserved a higher rating, since I now possessed the background knowledge necessary to enjoy it fully, lol.