Willow [柳 - やなぎ, yanagi] is the name of a suit in traditional hanafuda decks. In the standard Japanese ordering, it is generally taken to be the eleventh suit, representing the month of November [十一月 - じゅういちがつ, jūichigatsu] or the numeral 11. Originally, the cards in this suit all featured willow trees with hanging branches. There is one Chaff card, one Plain Ribbon, one Animal, and one Bright.
The Willow Chaff is known as the Lightning card [鬼札 - おにふだ, onifuda, lit. “demon card”], and in the modern Hachi-Hachi-Bana pattern the hanging willow branches on it have been altered to a more abstract grey and red cyclone shape, with various details (such as drums, windmills, a demon’s claw, etc.) underlying the coloration, depending on the specific deck.
The other three cards of the suit have maintained the willow tree motif. The Animal card features a long-tailed, red and yellow Swallow [燕 - つばめ, tsubame] flying between the drooping branches. The Bright card is generally known in English as the “Rain Man,” and features a human figure holding an umbrella under a willow tree beside a stream. In modern Japanese versions of the Rain Man card, the person depicted is prominent Heian era calligrapher Ono no Michikaze [小野道風 - おののみちかぜ], also known as Ono no Tōfū [おののとうふう], observing an orange frog in the lower left corner of the card.
In a few games, including Tensho, an alternate month ordering is used, with the Willow suit representing February or the number 2 instead. On the other hand, in the Korean suit order, Willow is considered to represent December or the number 12.
In multiple games, cards of this suit have special or different mechanics compared to cards with the same ranks or properties in other suits.
¶ Rain Man
The Rain Man is a lesser bright, with Bright yaku that include the Rain Man being generally worth less than the corresponding version without the Rainman. In Koi-Koi, it cannot form part of a “Three Brights” yaku.
Other designs (mainly older ones) depict someone running inside a closed umbrella, possibly an Amefurikozō or a Kasa-obake, two Japanese yōkai (demon-like supernatural beings in folklore) connected to umbrellas.
¶ Swallow
The Swallow is a basic Animal card that (comparatively) does not contribute to many yaku in most popular Hanafuda games.
While it is used as part of a “Small Birds” yaku in Musashino-Bana and Taiyaku, interestingly, the Korean Go-Stop and Minhwatu do not recognize it for the “Five Birds” yaku, analogous to how the Rain Man does not count towards “Three Brights” and “Four Brights” in most games.
In Koi-Koi, it can of course be used for the general “Animals” yaku.
¶ Plain Ribbon
Just like the Swallow and the Rain Man, this Ribbon is treated as lesser in many games.
It cannot be used with the other red Plain Ribbons to form the “Grass Ribbons” yaku found in e.g. Hana-Awase, Sakura and Bakabana.
It does not count towards the “Seven Ribbons” yaku in Hana-Awase and Hachi-Hachi either, though depending on the rule variation, it may award additional points if you capture it in addition to seven other Ribbons, similar to how the Rain Man does not help to make “Four Brights” in Hachi-Hachi but still makes for an upgrade to “Five Brights” if you capture him.
In Koi-Koi however, it can be used for the general “Ribbons” Yaku.
¶ Lightning
The Lightning card is the only Chaff card of Willow which is a special case since every other suit has two Chaffs, except Paulownia, which has three.
The special design and coloration this card has in most decks is likely due to its usage as a “wild card” or onifuda that can capture any card regardless of its suit (with some restrictions) in several games like e.g. Sakura, Bakabana or Mushi. For the exact mechanics of this, check the respective pages of the games.