Zack appears in Before Crisis -Final Fantasy VII- as a boss, and in Crisis Core -Final Fantasy VII- as the main (and only) playable character. He is capable of using various Materia and has mastered Materia Fusion. As a member of SOLDIER, he wields a large sword.
Zack is fought as a boss in Chapter 19 when the Player Turk intercepts him on a beach after getting orders to track down an escaped research sample. The Turk is shocked to discover the subject is Zack, and despite Zack pleading for the Turk to not turn him in, the Turk fights him. In the battle Zack runs around the battlefield evading the player's attacks rather than attacking him/her, and after a while he runs away.
The player catches up with him and another battle ensues, but Zack still won't attack the player, instead trying to convince them to his side. The second battle ends the same as the first one, with Zack running away. The player is attacked by an Adamantaimai, and after defeating it the player will follow after Zack who has reunited with Cloud. After seeing Cloud's Mako poisoning, the Player Turk will not fight Zack again, and will call Tseng that s/he has lost the targets.
Zack starts as a SOLDIER 2nd Class and has the usual abilities expected of a SOLDIER member. Him being personally mentored by 1st Class SOLDIER member Angeal, and briefly by Sephiroth, hints toward Zack being a cut above the other 2nds. Early in the game Zack is promoted to 1st Class. As well as being a master swordsman, Zack has mastered the art of Materia Fusion and the use of Materia, and is a skilled shot with a sniper rifle.
The player can permanently boost Materia's stats through Materia Fusion. The Materia can be fused with various items that grant stat boosts and the boosts carry over to different Materia during fusion. The Materia can then be equipped to Zack. Zack also gains minor stat boosts himself by leveling up by defeating enemies and having the DMW roll 7-7-7 during modulating phase.
Zack is a unique character in the Final Fantasy VII universe in that he can copy and adapt the Limit Breaks of other characters upon meeting them, and is thereafter able to perform those Limit Breaks as his own. The Crisis Core Complete Guide guide gives two examples of this: Zack adapting Healing Wave from Aerith's Healing Wind, and Meteor Shots from Cloud's Meteorain. This is how Zack can copy the signature attacks of Sephiroth and Genesis, Octaslash and Apocalypse. Zack's own Limit Break is Chain Slash, a quick sequence of five sword attacks. The DMW can perform summons through Summon Mode and Chocobo Mode.
Zack wields four weapons. As a SOLDIER 2nd Class, and briefly after his promotion to 1st, he uses a SOLDIER sword, the traditional weapon of most SOLDIER members. Zack later inherits the Buster Sword and in Costa del Sol, Zack fights briefly with a parasol, swinging it around like a sword, but for gameplay purposes it functions just as well as a weapon as the Buster Sword. Later on, in the mission after Nibelheim, he uses a sniper rifle.
The higher in ranks Zack climbs, the more accessories he can also equip.