(Shadow did actually have a redemption in the FS manga, if Lani would know about that haha. Though he does look a lot different here than he does in the manga. I can DM you pictures for a comparison if you want, as well as any clarification about Shadow you might need)
Sky squeezed Lani's hand when she grabbed his, in an attempt at some small comfort, then the handle of the knife she gave him. He narrowed his eyes at Ghirahim's remark. "Yes, I can't see why your attempt to sacrifice my girlfriend would be reason for me to dislike you," he shot back, not even realizing the absolute field day Ghirahim would have with the knowledge that he and Sun were actually a couple. "Forgive me for being irrational in not absolutely adoring you."
"…Yeah, okay, fair on both counts," the stranger said with a shrug. "Guess I'll start talking, then. My name's Shadow–well, my actual name is probably something way different, but that's all I can ever remember being called, so it's kinda sticking for now. Anyways. Don't freak out, but, uh, well, I used to work for Ganon–I don't anymore, though!" Shadow put his hands up in the air. "Honestly, that son of a bitch got off easy… Uh, sorry. Long story short, Link–you know him as Four–had kind of helped me realize that, 'Oh, shit! These guys suck and I shouldn't be helping them!' and so I helped the–him defeat Vaati, one of Ganon's big henchmen. Actually almost died in the process." Shadow frowned and paused for a second before shaking his head and continuing.
"Next thing I know, some bastard calling himself 'Dark Link' is telling me he summoned me to help him wipe out every incarnation of the Hero across time in order to resurrect an ultimate form of Ganon or whatever. The plan was to lie low, get whatever information I could and sabotage anywhere that wouldn't get me caught, then forward that information to you guys. But things changed. This guy wanted in on it, and then whatever the hell this is happened, and now we're here." Shadow looked over at Ghirahim. "I think that's pretty much it? You got anything you wanna add?"