No, Mana does not kiss Atem.
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They never did.
It is unconfirmed by Konami or 4Kids if they were truly "in love". Although, it is known that Mana was a very close childhood friend of Pharaoh ("prince" at the time) Atem. They were often known to be mischievious in their childhood.
I don't think Atem has a girlfriend because there is Anzu and Mana. He likes both girls. So, I don't think he has one.
He only sees them as friends.
They never kissed.