Sol Kuonji is one of the 9 siblings in the Kuonji family. Sol and Marè are twins and the only two confirmed related by blood
Sol so far seems childish, mischievous and free spirited. Upon debuting in the manga, he jumps out from behind a tree onto Tsukito and asks Stella who Tsukito is to her and if he can kill the boy. He also pays close attention to the smallest of movements, like when Mare stole Stella's handkercheif despite the latter not noticing at all.
In Sol's home country they were presumably from a family of three (a father was never mentioned). One day Marè was attacked by a stray dog and bitten baddly on the face, it is shown in the flash back that Sol was there but kept his back turned as if he didn't see nor hear anything, as if he didn't want to get involved.
Finding Marè's dead charred body, Sol refused to accept it due to the fact he never fufilled his duty as a 'big brother' to project Marè. To the point where he becomes seirous in this 'killing game', since if he wins he can get to successfully revive Marè.