Countless anime series depict their heroes and even their villains getting stronger over time, one way or another. Some anime characters gradually get stronger over time, while others suddenly gain immense power or a new form relatively quickly. In fact, some characters totally transform their abilities overnight, an instant upgrade that changes the game. Anime fans often call these a glow-up, related to the idea of someone remaking their personal appearance for the better.
Source：https://www.cbr.com/best-anime-glow-ups/
