Suguri's Lacie partner, Alto is the embodiment of an old music box that came to life as a Lacie. The music box had a design on it that showed two rabbits—foreshadowing for the future. Lacies come to life in order to fulfill a need for someone going through hard times. Because Suguri has two strong, competing needs, Alto's personality fragmented in two.
ALTO & MEZZO
The first personality is "Alto", his kind, supportive, and positive persona. This is the original personality, which came about when Suguri was a small child feeling pressured by and stressed because of his family. On a day when 9-year-old Suguri was so upset that he broke his violin, he later felt incredible remorse. He clung to a music box that his mother gave him, listening to the song as he cried. In his heart, Suguri was calling out for help. Alto came to life in order to answer that call.
The other personality is "Mezzo", a darker and more violent persona that is quick to anger. This personality supplements Suguri's fixation on revenge and his deep, deep hatred. Mezzo always existed within Alto, but lay dormant while Suguri did not need him. There were times even before Suguri's death when Mezzo emerged briefly, fueling anger and egging Suguri on, but these times were rare. After Suguri's death, Alto could not deal with the stress and pain. For the years until Suguri's resurrection, Alto lay dormant while the Mezzo personality took control. Alto has very little memory of what went on during that time, but knows that it was very painful.
The two personalities are constantly in conflict, putting a lot of stress on Alto. If both of his eyes are blue, then the Mezzo persona is in control of the body. When Alto is in his neutral state, he exhibits heterochromia. At times when Alto is very emotional, you can hear the faint sound of his music box heart playing a mournful tune.
Despite Suguri no longer being a child, Alto has chosen to stay with him in order to support him (Alto) and help him achieve revenge (Mezzo). Alto is fiercely loyal to Suguri, showing more and more yandere tendencies over time. His sense of self-worth is directly correlated to how Suguri feels about him. When Suguri is in a bad mood, it's more likely that Mezzo will eventually appear. Mezzo's manifestation often serves to relieve Alto of some of his stress and fear, as Mezzo can say and do things that Alto cannot.