Question

Who created Pokémon Showdown?

Answer 1

The other answer already tells..... but I am going to provide the source from Showdown!:

Owner
Guangcong Luo [Zarel]
– Development, Design, Sysadmin

Source

Answer 2

Pokémon Showdown was created by Zarel.

Source

Comments

Who's Zarel?

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You don't know? He's name shows in Teambuilder

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His Smogon account: https://www.smogon.com/forums/members/zarel.102803/
His Github account: https://github.com/Zarel