When she walked away, she made that spiteful "Tch!" sound in order to tell the friendly Marshmallow-kun not to pity her.
For an anime for kids, it was filled with so much awesomeness and surprises...So she intentionally made everyone hate her so that Marshmallow-kun would have a chance to stand out and be loved, then she walked away.
The voice actor's performance was awesome too.Although she wanted to be friends with everyone, Akemi knew that the hero, Marshmallow-kun, wanted it even more than her... In fact, that was one of the first lines he said in the show.
Awesome scene, too.
