Not to be confused with 'Serina's Transformation', which is technically when Serina transforms into her battlesuit.
Warning! This will contain spoilers!During Serina's bond arc, or more specifically her bond events, Serina is seen to take on a different figure to make her appear more mature and curvaceous. These events occur during Mysterious Magic, when at that incident, Serina was looking for a way to transform, out of jealousy towards her sister, Nazuna and generally because she wanted to prove she was capable of protecting herself.
Later through Serina events, such as Kiss and Transform Serina learns she can't use her star energy, meaning she can't use it for battle, to her dismay. Also that the effects wear off after an hour, which she can reject early to transform back to her normal self. Though more importantly, she can only transform if she preforms the spell she learnt, has someone with a high amount of either and is kissed by contact on the skin-though it wasn't specificed on what area.Although initially enjoying the form, Serina has found trouble with her transformation because of having to revert back to normal. Her final uses of her form occurs during the event: Pageant Prelims and Monstrous Beauty which is when she takes up a beauty pageant in hopes of returning the Academic park. Though after a monster breaks out from the Zoo and starts attacking the public, Serina reverts back to her old self and uses her physcial body to punch the monster until it is defeated.