Shirogane no Soleil is the very first Soleil game, and it begins with Ryuuhei - the protagonist - encountering Soul Valkyrie in the ruins of the Ship of Time in Iceland (fictional). There, he contracts with her in order to save his sister and her fellow archaeologists. However, as a result, she constantly drains his lifespan (literally the time he has to live) away in order to use magic, fight, and even just to exist on the mortal plane of existence. Unfortunately, at the same time he discovers that she is really immature as a Valkyrie... when she is barely able to fend off a mere Berserk (a fallen Einherjar from the world of Asgard, which perished long ago), showing off her immaturity and ineptness.
In addition, within Soul exists Hagalle, an older-seeming Valkyrie who is rather obsessed with Ryuuhei and pretty aggressive... which makes for some interesting clashes with his childhood friend and fiance Miori (who is your typical drill-hair tsundere ojousama with a side of impulsive violent behavior). Ryuuhei is... a natural philanderer. I really can't defend him there. He is the type who gets into relationships with women without really thinking about it and doesn't really think there is anything wrong with it... but hates the resulting jealousy and catfighting that results like the plague. He also lies like a rug and makes excuses when lies don't work to distract the girls from his faults.
That said, he is also brave, insanely protective of those he loves (though he can perhaps be said to love too many women romantically, hahaha), and is a disciplined warrior with a good sense of situational awareness.
This series, typical of the Soleil series as a whole, is twisty, full of character corruption, fallen deities, hope followed by despair followed by hope, and plot twists that make even experienced chuuni-lovers blink in surprise at times. It also is deeply steeped in Nordic mythology, though with its own fantasy twists.
The character relationships are surprisingly deep, considering the kind of situation the characters are in, and there is essentially only two endings, one of which is shorter and somewhat truncated/incomplete (Soul's), and one that is immensely satisfying for those who fell in love with the characters and setting (Hagalle's). Really, there are no separate heroine endings, but the walkthrough calls them the Soul and Hagalle endings.
Did I enjoy this game? Yes, I did so immensely. It also helped me make sense of a lot of the background in Shin Shirogane no Soleil, and I'll probably end up re-rating that one after I replay it as a result. For those who like the darker side of chuuni, this is a good game to look into, and it is also attractive for people who like Norse Mythology based stories.