Shadow Knife (シャドウナイフ; shadou naifu) is an Ostinato Musician who writes songs to µ, and serves as her bodyguard due to his combat abilities. His real name is Daiki Yamada.
While it is unknown what Shadow Knife looks like in the real world, in Mobius, he took on the name and appearance of a justice hero from an anime. His hair is silvery white and his eyes dark gray while his skin is pale. He wears a gray coat with a fur hood, a black mask over the lower half of his face which also covers his throat, a white top above a black one, a black kilt with black and white patterns in the middle. He also wears black leggings and black knee-long boots. Additionally, he hides two daggers in his coat.
Shadow Knife is seemingly obsessed with a distorted justice he wants to force upon others. He sees himself as the one and only bearer of justice and wants to "destroy evil". He wants to bring justice and punishment upon people he deems as evil. Having been bullied in middle school, he seeks revenge with his so called justice and makes people atone for their "sins".
"Also... the Hidden? Couldn't that be something even you don't want to know?"
The following content contains spoilers. Please proceed at your own risk.
Not much is known about Daiki's past, aside from the fact that he was the school punching bag in middle school. He would get tied to a pole or thrown into a locker while people banged on it, and he was also forced to eat rotten food. At some point, he listened to one of µ's songs, which brought him into Mobius. There, he took on the name, appearance and powers of a justice hero from an anime to wreak vengeance upon those who did wrong to him.
Shadow Knife is fought multiple times in Landmark Tower. On the rooftop, the Go-Home Club find μ and Shadow Knife. After Thorn comes to take μ away and the Go-Home Club defeat him, Shadow Knife falls over the edge of the tower to his death.
If you choose not to take Thorn's hand to do the musician route, he falls off Landmark Tower, just like in the original game.
In the Ostinato Route of Overdose, depending on your decisions, the circumstances surrounding how he survives change.
If you don't finish Eiji Biwasaka's character episode in time, right before Daiki falls, Kotaro Tomoe takes his hand, saving him, at the cost of Kotaro's life, mentally scarring Daiki.
After the Go-Home Club's encounter with Wicked, Shadow Knife joins the Protagonist, along with Ike-P and Wicked as they investigate the Sea Paraiso Aquarium. There, Kensuke Hibiki, Eiji Biwasaka, and Aria wait. Kensuke blames Shadow Knife for Kotaro's death at Landmark Tower causing Shadow Knife to have a mental breakdown.
After returning from Sea Paraiso, Daiki is put on a team with the protagonist, Wicked, and Thorn, where on arriving to Landmark Tower, he remembers Kotaro's death again. Thorn brainwashes Shadow Knife so that Shadow Knife blindly believes Thorn.
If you are able to finish Eiji Biwasaka's character episode before the deadline, then Shadow Knife falls off Landmark Tower, like the original game, where he gets too close to the edge then falls. However, when you return to the musicians room after the Wicked incident, you'll find him, alive, shaking in fear, after he crashed through a ceiling, during his fall, leaving him just barely alive until μ was able to help heal his wounds.
In the Leave end, if Daiki is alive, then he's seen passing by a group of people bullying a boy. He then passes a poster advertising the Shadow Knife anime, then turns back to save the bullied boy.
In the Stay end, Shadow Knife is seen during the graduation/entrance loop, where one by one, each character dies due to the Astral Syndrome.