Tsukito is a supporting main character related to one of the family butlers, Kokonoe.
There seems to be two sides to his personality. He's usually awkward and unintentionally acts a little stalkerish. Though at times he can act quite calm and collected. He's also very loyal to Stella though his connection to her is initially unknown. He's more than willing to lay his life on the line to protect Stella if the situation calls for it.
Earlier in the story all that's known about Tsukito is that he is Kokonoe's sister's grandson. Later in the story it's revealed that Tsukito is the real personality of Zeno.
As a child, Tsukito was constantly threatened by spirits, therefore he created a second personality that, unlike him, couldn't see spirits and was able to live a normal life. He met Stella and got closer to her, becoming her beloved "big brother", and was the one who protected her during the airplane accident, obtaining a scar on his left hand that matches with the scar Stella has in her right hand.
Zeno was stabbed by the "white haired boy" that used to bully Stella in the orphanage during the Mad Tea party, and had his identity stolen by him. Before he died, he was saved by Kokonoe, who managed to keep him alive, but his hair became white like the boy's who tried to kill him used to be.
He dedicates his life to protect Stella, and for a long time struggled with the fact that he couldn't tell her that the one she believed was her big brother Zeno was actually a phony.
- Tsukito's kanji "月兔" reads "moon rabbit" a reference to the folklore about the rabbit in the moon with mortar and pestle, another referencing the white rabbit Alice follows in Alice in Wonderland.
- He seems to be able to collect and somewhat control spirits, as well as see them.
- his Alter ego is presumed to be the 'white rabbit' or well Alias, as hinted a few times in volumes 1 & 2.