Zook first appears at Gateon Port near the beginning of the game. After Jovi, the player's sister, accidentally runs into him, he begins to threaten her with his ShadowZangoose. However, Mr. Verich, Eldes, and Ardos appear at the scene, and Ardos proceeds to easily defeat Zook's Zangoose with his powerful Alakazam.
Later the player encounters Zook at the Cipher Key Lair, where he's standing outside of the facility for guard under Gorigan's orders, as punishment for "showing off Shadow Pokémon without permission". He's first fending off a couple of Team Snagem members, before he notices the player, and challenges him to a battle. Because the player's Snag Machine was stolen by Team Snagem at the S.S. Libra wreck, he will be unable to snag Zook's Shadow Zangoose. After the player has recovered the Snag Machine from Team Snagem's hideout, he can battle Zook again. After Zook is defeated once more, he will leave "somewhere far away where Cipher won't have any influence", never to be seen again.
Due to Michael's Snag Machine having been stolen, he will be unable to snag Zook's Zangoose during the first battle against him. After the Snag Machine is recovered from Snagem Hideout, Zook is rematched with the same team as before.
"You're calling the dashing and terrific Mr. Zook a lunk? How dare you! Grrrrr! You're not getting away with this!"
After defeating Biden
"Hey, you want more of where that came from?"
"You're not running off with them? You've got guts, but... Huh?!"
"Wait a second! You're that brat who was at Gateon Port! What are you doing here?!"
"You're nothing but bad news! Ever since I saw you, I've had nothing but the worst luck! First, at Gateon Port, some guy challenged me to a battle out of the blue and promptly stomped me flat. Then Gorigan gave me one major dressing-down for showing off Shadow Pokémon without permission. Because of that, he made me stand guard here as punishment."
"There's one thing I can't figure out, though. How'd Gorigan ever find out about me anyway?"
"Gaaah! Remembering that is getting me riled! This time, I'll flatten you!"
Upon being defeated
"But... This can't be happening!"
After being defeated
"You're just some kid! How could I, the mighty Zook, get beat by just some kid?! Because you fluked out! You only won because you got lucky! That's got to be the answer! I won't battle you again. If you win by a fluke again, my reputation will take a beating!"
If talked to
"Get lost, punk! If I, the mighty Zook, get serious, you'll be crushed in no time!"
Cipher Key Lair (second visit)
"Quit scurrying around! You're getting on my nerves! Don't think you're hot stuff just because you beat me once by a fluke! And what's with showing off that weird machine on your arm? You're begging for a thumping, and I'll deliver it this time! I'll bash, dash, and crash you, just like you want!"
Upon being defeated
"Wroooar! What'll I do? What'll I do? Gorigan's going to chew me out again!"
After being defeated
"Urrrrgh... The punk thumped me... And Gorigan will be furious with me again... ...I should go away. Somewhere far away where Cipher won't have any influence. So long, punk. I've had enough of battling you to last a lifetime."
The gender of Zook's Shadow Zangoose is not set in stone until it is sent out against the player at the Cipher Key Lair. Due to appearing twice before such a point, this particular Zangoose can potentially change its gender up to two times over the course of events. An example of this might consist of Zangoose's being female when facing Ardos at Gateon Port, male when facing Biden at Cipher Key Lair, and female again when facing the player at Cipher Key Lair.
Despite Zangoose and Seviper being mortal enemies, Zook has both of them in his team, and it is possible for both of them to be out on the field at the same time.
In the demo version of Pokémon XD: Gale of Darkness, Zook has a Shadow Shellder on his team. In the finished game, Cipher Peon Gorog is the character who has Shadow Shellder.