Teito's heart bleeds... and the power of Mikhael makes him a sort of schizophrenic, because another existence lives into him.
When Teito looses the control for some strong reasons, Mikhael takes his place in Teito's mind and reveals his existence. Teito's eyes become red like blood and his expression becomes evil and sadistic.
It's because Mikhael is the God of Destruction, and he's located in the "Eye of Mikhael", a jem that Teito's father hid in his right hand when he was a baby. The power of Mikhael is the one that only the successor of Raggs Kingdom can hide in his body, and since that moment Mikhael was sealed like in an eternal dream.
The positive thing is that Mikhael can feel Teito's feeling and for this reason he respects them, loving who he loves and hating who he hates.
An example is the thanksful that Mikhael gives to Hakuren to be friend of Teito, or the desire of killing Ayanami, Teito's enemy and killer of his father and best friend. He's very loyal to his master and maybe his fear is to be alone, because Teito feels his sadness in a prove for the exam to be Bishop.
With the development of the story the Eye of Mikhael is lost but he continues to lives into Teito, because it's the destiny of the prince of Raggs Kingdom.