Though no direct explanation was ever given about it by Strider, it is assumed Dark Strider is PhantomStrider's "dark side." Dark Strider falls into the TV Trope of "Superpowered evil side."
Dark Strider wears a dark blue shirt and has an over-abundance of black eyeliner/paint on his face. He tends to have a more gravelly voice than Strider and generally seems to appear from Strider when he is angry. He is typically more harsh and angry in his temperament than Strider would normally be.
He often appears when Strider is attacking something with his saber (e.g. a Boo or his computer). However, Dark Strider typically uses his hands in abnormally fast slashes to attack instead of using his saber. A black liquid can sometimes be seen smeared on his hands.
Dark Strider's first appearance was from a hidden video "LiveStrider Short" called simply "Dark Strider" where he transforms and cheesily fights about 50 ghosts.Hidden video link:https://www.youtube.com/watch?v=1TqzV7IwFeA
Dark Strider is loosely based of Sora's AntiForm from Kingdom Hearts 2, representing the lingering hatred within Strider.