Rhian is the Good identical twin brother of Rafal, Since they are twins it can be presumed that he too had white hair and bright 'glacier' blue eyes. 200 years before Sophie and Agatha's fairytale, he was killed by his own Evil brother in the Great War. Though not much is known about him, it is stated on page 220 of the book, A Student's Revised History of the Woods:
"Neither of the proper names of the Good School Master or the Evil School Master was ever known, though in my dealings with the latter, I can attest to hearing the Evil School Master - now known as Rafal - casually refer to his own brother as 'Rhian'. Whether that was the Good brother's name or merely a slip of the tongue, we'll never know."
His love for his brother kept him immortal and alive and his brother's love for him kept Rhian immortal and alive.
In the 1st book, at the mural, it is shown that he and Rafal looked very similar and shared the same hair colour : White.
In Book 3, Sophie asks what was the name of the Good School Master, Rafal replies that how will the name of his own Good brother help her to get to know him better.