Gumball and Penny had always had massive crushes on each other, but the two of them had always struggled to express their feelings, that is, until the events of the episode "The Shell," when Gumball finally confessed his feelings for her and kissed her, and the two have been dating since then.
She was part of The Treehouse Girls until it disbanded due to the tree being cut down by Tobias and Banana Joe and is currently on the school's cheerleading team.
In Season 3, the episode "The Shell" revealed that Penny underneath her shell is a shape-shifting, fairy-like creature.
She can transform into any animal, and every time, the animal has a set of antlers.
"The Storm" showcases this trait again, as she has nothing but cynicism after she is misled into believing Gumball intentionally kissed Carmen.
After the events of "The Shell," Penny remains in her fairy-like form. She sticks up for her friends often, implying she has a strong sense of integrity.
She also showed no anger when Darwin made several attempts to sabotage her relationship with Gumball, but rather sympathized with him.
She is one of the few people to understand Gumball, whom she has a massive crush on.
In her early concept, she did not have a visible mouth and her antlers were darker.
In Season 1, Penny had a straight, round shell similar to her father's, with noticeable holes where her arms and antlers came out of.
After Gumball helps her come to terms with herself, though, she has become more open and enthusiastic than she was previously.