Ishizu Ishtar becomes available character by completing unlock missions. She appears in the Gate after you finish a trigger event, then you can have a duel against him and challenge missions to unlock her.
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Summon LIGHT-Attribute monster(s) 200 time(s) in total
It does not count Set, Flip Summon, and Special Summon (unconfirmed).
Summoning a Light-Attribute monster in PVP Duels including, Ranked Duels, Casual Duels, and Friend Duels, is counted (unconfirmed).
Ishizu unlock missions
Win a Duel against Ishizu Ishtar with at least 6000 LP remaining.
Win a Duel against Ishizu Ishtar at Level 30 in Duel World.
Complete all Character Unlock Missions.
Stone of Spell x3
Let's use Light monsters from the beginning
Here is a list of Light-attribute monsters that are easy to get and available from the early stage of the game. Build a deck that contains those monsters and summon them as much as you can even in the initial phase. See also the following 2 tricks:
Do not set, flip summon, or special summon them. It is said that only normal summon is applicable although that is unconfirmed.
If your Monster Zone is full with LIGHT monsters, conduct tribute summon. Or switch their battle position to defense, then standard duelists will attack your monster that have low DEF, and you can summon the next one.
Set LP Boost skills to your deck and use cards that regenerate your Life Points to complete one of Ishizu unlock missions, win a Duel against Ishizu with at least 6000 LP remaining. Joey, Tea, and Mai can acquire LP Boost skills by leveling up, and other also can acquire them as drop (duel reward) skill.
[LP Boost α] Increases starting Life Points by 1000.
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... the author of this wrote " him and challenge missions to unlock her." calling him a boy then girl, but the gender is known to be a girl currently and likely forever(unless konami decided to change her current gender) :P