They are exactly what they sound like, they're bots that only exist to AFK in matches.
They don't do anything else.

Why are they doing that?
They're doing that to get ranks, most people get annoyed with them and just surrenders on spot, and eventually enough people surrenders to make them level up a rank, + they can do this 24/7 so no matter how long does it take, they'll rank up eventually.
And then the leveled up IP can be sold, or used by bot maker.

Why are people impersonating them?
Like mentioned above; people usually surrender on them, so if you see someone who looks like bot, there's high chance you will want to skip it, however, some people can treat afk bots like free wins, and that's where impersonating comes to play. Since afk bot does nothing, you just need to play one card, end your turn, and wait for your opponent to get timed out, (some bots also disconnect after a while). And while you play only one card bot impersonators are free to go through their entire combo unstopped, and destroy you.

How to spot AFK bot:
Afk bots always have:
-Default icon
-Rescue Rabbit mate
-No cosmetic additions
-Name written in japaneese (usually short name)
-They always let you go first
-They always use one of stock decks, so they have no hand-traps
-5 Cards in extra deck
-They never leave draw phase (If they go second)

If any of these things isn't on place there's high chance you're going against impersonator.

If you're the patient type and use bots for free wins, here's one simple method to defend against impersonators:
Just go along with your combo, if you're going against bot you'll be bound to lose few minutes anyway, however if you go agains impersonator, they may try to stop your combo, or they can even surrender on spot. Don't just settle with single card, play like you would normally.

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