Azuki beans. Vigna angularis. 小豆。Also written in English as "adzuki."
Small (5 mm) dark red bean, mottled varieties also exist. Written as "small bean" in contrast to soy beans which are written as "large bean." Azuki beans were already cultivated in China about 3,000 years ago and were later brought to Japan. Second most popular bean after the soy bean.
[Image from Wikipedia]
Azuki beans have a mild and delicate flavor. Azuki are cooked with rice and also form an ingredient in sekihan, glutinous rice steamed with azuki beans which give the dish a red color. Sekihan is consumed on auspicious occasions, such as birthdays and weddings.
Azuki are also commonly boiled with sugar to make red bean past, an. An is used as a popular filling for wagashi, Japanese sweets, such as for example yokan. An can be either sieved (koshian) or unsieved (tsubuan).
A more liquid version of an is also used to make shiruko, a sweet soup with toasted mochi or dango. Unsieved an results in that case in zenzai.