Roxas looks exactly like Ventus the Youngest of the Chasers which Scholars fear is the return of Vanitas as the omens dictate in the return of Xehanort, fearing this also, King Roland Twilight gave his son the updated relics belonging to Ventus including Ventus' armour and Keyblade.
Roxas, as Riku puts it, has a strain of Nobody Syndrome. Unlike the enemies, Nobodies lack emotions but emulate them from past memories, those with this Syndrome however have actual emotions but refuse to act upon them due to traumatic experience in their past and/or memories.
Roxas' personality is detached compared to a normal person and his closest resemblance in the Dark Empire would be Yami Tenebrae whom isn't so detached but is instead calm and lazy, these factors are also due to Roxas being shipped around to Fight Heartless and the like while a part of the Twilight Kingdom's Army.
Roxas was born into the Twilight Kingdom as its only prince and therefore Crown Prince which his birth caused much celebration even at the cost of his mother.
Looking upon Roxas from youth the people began to wonder at the likeness between Roxas and the legendary Chaser of Twilight: Ventus the Youngest.
This caused a number of Scientists and scholars to forbode the possibility of Roxas being the seed of Vanitas' return, whom if he returned would become one of the many Omens upon the land marking the return of Xehanort.
Roxas spent most of his youth in the only way he could think of to make up for his abnormality by looking like Ventus the Youngest, to this end, Roxas joined the Twilight Kingdom's Army and fought against the Heartless in the North while seeing dramtic faliure time and again.
Roxas would later witness the rise of the Unversed, one of the Omens dictated in the Return of Xehanort specified in the legends and Roxas was intent on fighting and defeating the elite Heartless, needless to say that it was a Win/Lose Situation as not all of the Army had a chance against their Normal Enemies let alone let by an Elite "elder brother".
Chain of Dark Events
Roxas was sent as Crown Prince to represent the Twilight Kingdom in Sora's coming-of-age celebration