Bandit Keith becomes available character by completing unlock missions. He appears in the gate after you finish a trigger event, and you can have a duel against him and challenge missions to unlock him at that point.
Win duels 50 times against legendary duelists.
Collect 2 Machine-Type Cards.
Win 1 Duel(s) against Bandit Keith at Level 30 in Duel World.
Complete all Character Unlock Missions
Stone of Flame x3
4 ways to collect 2 Machine monsters
Here are 4 things you can do to complete one of Keith unlock mission: Collect 2 Machine-Type cards.
1. Win a duel against Keith
You can obtain Machine-type monsters as reward for winning duel(s) against Bandit Keith. When his level is 30 or more, he can drop 5 different Machine cards.
Since one duel reward is given to you per 1000 score, getting high score each duel is important to save gate keys that is used to having duel(s) against Keith and Joey and to increase Machine-type cards drop rate.
You can trade Cyber-Stein with trader items by visiting Card Trader. Collecting required trader items for the card is not difficult for its rarity: Rare. You however may have to wait to see the card for some days.
I unlocked Keith after I beat one of the casual duelists in the duel world, and it said "Beat 50 Legendary Duelists unlocked". Then I got Keith. I wonder how those casual duelists became legendary. Haha