League of Legends (LoL) Question: Master Yi in challenger?

You can theoretically make it to Challenger using any 1 champion.
That being said, it's much easier to have a group of champions that you use rather than just using 1 (although I've heard when climbing to plat, a lot of people find it easier to main 1 champion).

yhea some do just search up cowsep on youtube he only plays yi ^^

Sure, it's possible. It will take a long time though, and high-elo players can counter yi pretty easily. Just cc him and he's pretty much dead

Some people have made it pretty high up the ladder using only one champion i.e. Irelia Carries U, or Annie Bot

Do not play yi in ranked.

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