Rusalka

Japanese: rusalka

Other examples of deification of historical figures include the Eastern Slavic Kii, Shchek, and Khoriv, and the Western Slavic Tsekh, Lyakh, and Krak, who are heroes of lineages that became the founders of communities and appear in mythological epics. Others include the Russian witch Granny Yaga, the immortal Koshchei, and Old Man Moroz the Cold, as well as many spirits associated with natural and cultural phenomena that appear in folk beliefs (such as the Russian water spirit Vodianoi, the forest spirit Leshii, the house spirit Domovoi, and the water and forest spirit Rusalka). It is thought that these figures and spirits of folk tales were once larger pagan deities.

From Russia

...In order to ensure a good haul of fish without offending the vodyanoi, there was also a custom of offering sacrifices to the water spirits when they woke up from hibernation. The rusalka was also a water spirit, but some considered her to be the wife of the vodyanoi, while others considered her to have no connection to them at all. A rusalka could be a girl who had committed suicide by jumping into the sea or an infant who had died before being baptized.

*Some of the terminology explanations that mention "rusalka" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…　以上あげたもの以外に，歴史上の人物が神格化した例として東スラブのキイKii，シチェークShchek，ホリフKhoriv，西スラブのツェフTsekh，リャフLyakh，クラクKrakなど共同体の始祖となった系譜上の英雄があり，神話的叙事詩に登場する。またロシアの魔女ヤガーばあさん，不死のコシチェーイ老人，寒さのモローズ爺といった昔話に登場する存在，民間信仰に現れる自然と文化の現象にちなんだ多くの精霊(ロシアの水の精ボジャノーイVodyanoi，森の精レーシーLeshii，家の精ドモボーイDomovoi，水と森の精ルサールカRusalkaなど)もスラブの神話世界を構成する。こうした昔話の形象や精霊はかつてはより大きな異教的神格であったと想像される。…