Kasuga is a fictional ninja introduced in Sengoku BASARA. Her name is a reference to Kenshin's main fortress, Kasugayama Castle.
Kasuga was originally from the same ninja village as Sarutobi Sasuke, and the two had an amiable relationship. Eventually she was sent on a mission to kill Uesugi Kenshin. However, before she could kill Kenshin, Kasuga falls for the warlord. Kenshin pardons her and took her in as a spy. However, because of her failure to assassinate Uesugi, she was exiled from her village. From then on Kasuga served Kenshin and became his most loyal ninja. In Sengoku BASARA 4 Sumeragi, it is also said that it was when she entered Kenshin's employ that she "became human", due to the love she felt for her master- in comparison, Sasuke "became human" due to having Kasuga to worry about back in their village.
Kasuga often gushes about her lord Kenshin, going so far as to have "delusional" hallucinations where she calls out his name as she braces herself tightly. When these things occur, sometimes other notice and find amusement, while at other times those around simply brush it off. When alone around Kenshin and not on the job, she tends to be quite amiable- but can become very reserved when Kenshin dotes upon her. However, if Kenshin goes too far with his compliments, Kasuga may have one of her episodes. It is known that her love for Kenshin is such that she harbors a desire to marry him, as evidenced from her route in Sengoku BASARA 4 Sumeragi.
That being said Kasuga can be very dour, whether on the job or not. She clearly takes her job seriously and is powerful enough on the battlefield to be called a "goddess" by some Uesugi retainers. She isn't humorless, however, and has been known to act sassy towards both enemies and allies alike. She is not without kindness, however, and displays warmth towards those in the Uesugi army and concern for her allies. She in fact seems to loathe killing, only doing so in order to achieve the world Kenshin wants,
Kasuga has a rivalry with Sasuke, in no small part due to the friendship they shared. Sasuke often teases her, both about her skills and in a mocking romantic fashion, which Kasuga reacts to with great irritation. It's clear she still harbors some warm feelings towards him, however, as she does not hesitate to assist him at times when he is not acting against Kenshin. She also has a rivalry with Nōhime, due to the other woman's role as Nobunaga's love interest and protector, and their similar personalities.
Kasuga is known to be a very beautiful woman, often complimented as such by Kenshin. But she is noted by other characters to have a very suggestive appearance- much to her chagrin- due in no small part to her low-cut bodysuit.
In the first Sengoku BASARA she wields two small ringblades in a fashion similar to Sasuke's shuriken, but in later titles, she wields two sets of four kunai attached by a wire. In Sengoku BASARA 3 Utage, her personal item allows her to bind enemies with the wire- while, in Sengoku BASARA 4 Sumeragi, her attacks by default can bind enemies. Like Sasuke with his crow, she can glide down using an owl familiar.
In Sengoku BASARA 3, Kasuga only appears in one stage. In the Battle of Kawanakajima-Tundra, after the player defeats Keiji the first time and Kanetsugu Naoe, she will appear at the North-West side of the battlefield, opposite from a returned Keiji. As you fight her, she has unique lines for each character but will promise Kenshin's protection at least once. After her defeat, her lines presume she retreats instead of dies, with the exception of when you're fighting as Ishida Mitsunari. Once she and Keiji have both been defeated, Kenshin appears on the battlefield.
Kasuga is a playable character in Sengoku Basara 1 & 2. She is demoted to NPC for Sengoku BASARA 3, but is made playable in Sengoku BASARA 3 Utage. In this expansion, she does not have her own story, but appears once again in Kawanakajima-Tundra, as well in the Kyoto Fireworks Festival stage alongside Keiji. The player can unlock her for Free Mode and Unification Mode by clearing the stories of Sarutobi Sasuke or Tachibana Muneshige.
And she is reprised by Venus Terzo in the English dub and Natsuko Kuwatani in the Japanese dub.