
To be honest I’m not as interested to be as heavily invested as the main couple, so here’s a quick summary.
[spoiler]
Even as a kid Toguto firmly believed his existent was to serve the country. He met Suren when both were still small and it was decided that Suren would become his mate. The young, naive and and shy Suren grew up completely devoted to Toguto - he was unsocialised and never talked to others; Suren spent all his time around Toguto and constantly expressed how he wanted to quickly become his mate. Toguto was his whole world. On the other hand, Toguto treated their engagement merely as something that was going to benefit the country because it followed the tradition, therefore he would take part in it. Their one-sided relationship and affection clearly showed in how Suren came to visit Toguto everyday but Toguto never came to see him on his own.
One day Suren overheard Toguto telling Badoma (the other white hart lady in book 1) that love is not necessary and as long as the other is a white hart, they can be his mate. Heartbroken, Suren confronted him and finally realised that Toguto had no interest in him as a person. After this event Suren changed completely. He stopped coming to Toguto’s place and started hanging out with the maids and other people in the palace. There’s a rumour that he had been having different people accompanying him at night.
In response to this, the prince angrily accused Suren of breaking the palace’s rule that a white hart can only engage with royals. “Are you mad because I broke the rule?” “Yes” “Then it’s okay if my partner is a royal, right?”. Suren pushed Toguto down on his bed and... (no more memory) Toguto woke up from his dream of the old days.
(My guess here: Suren might have broken the engagement or it was cancelled because of his streak of indecent acts. He still remained in the palace as a white hart but not engaged to anyone. Toguto then received a new mate that was Alta).
Alta’s words before their parting stirred up something in Toguto and he thought long and hard about how Alta described Isle as someone he “could never forget no matter how much I want to”. To him, that person is Suren. They met in the garden one day and after many years Toguto finally apologised for being the reason of Suren’s rebels. They talked it out, Suren was still in love with Toguto and they decided to give it a try. I will try making you my number one priority (over my country), Toguto said.
There are someone more minor issues later where Suren struggled with what he did in the past (flirting with the maids and sleeping around were the only ways he knew to vent his frustration and anger) and Toguto not sure how to place their relationship in regards to his responsibility as a prince to his country. They slept together (no explicit). In the end things seems to be working out (even though Toguto still showed an emotionless face to his mate and I can’t see any chemistry between them .__.).

Urghhh This makes me despise Toguto even more. How can he be so emotionless and not thinking about the feelings of those around him like this. Poor Suren. He deserves a better person than Toguto... Well, at least things work out for them so I guess Suren is satisfied with that.
Thank you very much for the summary!! I really appreciate it!
I have finally read Toguto & Suren’s story seriously and I really can’t like Toguto. He’s lucky to be loved by someone else that much. He was cruel to the person closest to him and drove them to destruction then thought nothing of it until years later when Alta’s words finally woke him up. He’s way too dense and austere.