Vanitas's Background Story And Explaination

Ready for the final battle

If somebody is confused about how Vanitas come to look like Sora well I can explain. Vanitas had taken too much of Ventus’s heart when the first split in two. In the beginning, he looked like a humanoid-heartless with red eyes and no face. Ventus about to lose his heart is approached by a newborn heart (Sora) who shined his light on Ventus curing his broken heart. This caused Vanitas to copy Sora’s face due to Sora connecting with Ventus. His hair became black and his eyes turned yellow. His battle stand is similar to Riku just because since Ventus is connected to Sora, then Vanitas received different characteristics form the other connections Sora has made.

Vanitas loses his only chance

Vaintas can be very arrogant and ruthless and this is to be expected because he is pure darkness. But, he is not pure darkness (at first). In fact in the Birth by Sleep novel portrays Vanitas to be exactly like Ventus when they first split (four years before the actual game). He was timid, fragile and weak. Vanitas hated those words more than anything and blamed Ventus for it. Vanitas was by himself and very lonely. He became sad when Ventus left the Baldlands and wanted him back (despite that he would beat the hell out of him every time he saw the poor unconscious boy). Vanitas can feel feelings that Ventus can feel (being his other half of course), so he knows well of feelings like love, kindness and other emotions. Just imagine Ventus having a great time with Terra and Aqua while those warm family emotions are transferred to Vanitas who is left alone and is surrounded by nothing but lifeless keyblades and rocks.

Vanitas only half reunited with Ventus

After all those years Vanitas became ruthless and his feelings became darker towards Ventus so that the day when they fight, they can merge together ending his sorrowful pain. That is the Vanitas that players met in the game. 

***Warning: This information comes from the Birth by Sleep novel and some people may consider it non-canon.