Question

Why a person in my Acquaintances has a blue circle around of his face?

Answer 1

It means that the Acquaintance is currently using O-Power.
You can also get their current O-power they are using by tapping their character.

Comments

but O-Powers are Yellow Circles. not blue :l

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It could be blue

Answer 2

The blue circle means that that person used an o- power on you when you were busy like trading battling pr video those kinds of things. If you click on the avatar with your stylus, you get chose if you would like to exept the o- power or not.

I hope this helped