Kobato. Concepts

Angels are beings that reside in Heaven. They are often portrayed with wings, halos, or both.

Christmas is a holiday that is celebrated on December the 25th where people often exchange presents.

The practice of dressing like characters from anime, manga, video games, and even novels, TV shows, celebrities, etc.

Used frequently in Anime to either recap past events or provide a back story for a character, either their origin story or their off-screen training of new attack. Sometime and entire episode/chapter would be a entire flashback.

Japanese Festivals are referred to in Japan as 'Matsuri'. Festivals can be local affairs or even nationwide holidays. In many ways similar to Fairs in the US.

Reincarnation is a common theme in most Asian literature, and frequently a character will appear who turns out to be the reincarnation of a past figure (fictional or otherwise).

Rivals are competitor to someone. Unlike an enemy, rivals could have similar goals but disagree on the process.

Valentine's Day is a holiday celebrated across much of the world on February 14th. The practice of the holiday is much different in Japan then in the Western parts of the world.