Sidonai was born the middle child of his parents. He did have an elder sister, at one point, but she was murdered somewhere between Haradi and Dalmasca. Nowadays, Sidonai believes his sister was killed, either on accident or not, amid a scuffle with slavers, but he can't prove this and neither can anyone else. As such, he's never voiced this opinion, and, anyway, it doesn't matter now. His sister was set to take over Haradi after their mother left the throne or passed away, whichever happened first, but with her passing, that fell to Sidonai.
His leadership oriented education came somewhat late, a mistake he did not intend to repeat with his own children. He learned relatively quickly, at least, which helped immensely with how late he came into it. Still, he didn't exactly expect to end up taking over quite as early as he did. Sidonai was lucky enough to find real love at least once in his life, rather than love that only masqueraded as such, and he had two sons by her, before she too, left him. His mother left the throne to travel the world in her later life, not long after the birth of his eldest, Ryhil, and Sidonai became King of Haradi.
When his youngest son, Tihaan, was still yet a young boy, he developed a love for Haradi's people. Ryhil had expressed disinterest in becoming the next King, and Sidonai thought Tihaan would be well suited to it, given his often vehement insistence upon certain changes and regulations that were beneficial to the common people of Haradi. He had a much better concept of what they needed and wanted than most, and while Ryhil wasn't unintelligent and could have learned these the same, Tihaan already had.
Unfortunately, he disappeared sometime around when he turned 15. At first, Sidonai was content to let the royal guard do their job, and locate him on their own, but progress was never made. Sidonai did find slavers slinking about in back alleys in Haradi, and had them removed. His son remained missing long enough, Ryhil soon joined in the search, and Sidonai became intensely antsy about the entire ordeal. Antsy enough, he's rather tempted to leave Haradi in trusted hands, and travel to Dalmasca himself.
Still, he doesn't have anyone that he trusts his city to, and he'll likely remain where he is. When magic was still relatively new to humes, even before, to some extent, Sidonai was one of the first to begin toying with auratech, and most attribute him as the father of auratech, and the reason it is so advanced in Haradi. Over time, it became one of the staples and hallmarks of Haradi, and, despite knowing Suyis wasn't selling all the technological know-hows on airships, Sidonai still bought the designs for them. Haradi and Saqqara are now the only two places to find airships in use day to day.