Oichi is the younger sister of Oda Nobunaga and the wife of Azai Nagamasa, who made her debut as a playable character in Sengoku BASARA 2 Heroes. Ever since, she has made an appearance in all of the Sengoku BASARA titles to date. She is often considered as the most tragic character in the series.
Oichi is Depicted as a tragic woman with a supernatural power who is very melancholic and insecure. Weak-willed and depressed. She always thinks that she is guilty of things. She was first sent by her brother, Oda Nobunaga, to marry Azai Nagamasa and to spy on him. However, she genuinely fell in love with him and was torn between betraying her brother and her lover.
Oichi's story in Sengoku Basara 2 starts with she and Nagamasa fighting against the Oda army. After witnessing her husband's death at the of hands of Nobunaga and then being forced to fight on in Nobunaga's name, she slowly starts to lose her sanity. In the end, she succumbs to her own dark powers, which leads her to kill Nobunaga's retainers and eventually Nobunaga himself. After killing her brother in Honnō-ji, Oichi regains her sanity for a little while, then breaks down in tears at her atrocity to her family and clan. The temple collapses and she is presumed dead afterwards.
Oichi inexplicably returns in Sengoku Basara 3having lost her memories after suppressing the pain of witnessing Nagamasa's death and being forced to do cruelties under Nobunaga's name in the past. Rather than bearing the Azai crest, she now uses the Oda crest.
In her Red route, Oichi joins Ieyasu - Lord Happy - because he's kind to her and wants to always be there for her. She repeatedly joins him on the battlefield, despite his protests, and him asking her to stay behind. She chooses to kill Mitsunari - Lord Grumpy - to save him from the "hole in his heart".
In her Blue route, she joins Mitsunari and becomes a pawn for in Yoshitsugu's plans to bring misfortune in the world. In this path, her dark side is fostered and fed on the souls of those she is made to fight. Once Oichi defeats Ieyasu at Sekigahara, she turns against Yoshitsugu, bringing darkness to the world.
In her Green route, she follows a voice, leading to a battle with either Tenkai or Hideaki. Fighting Tenkai makes him admit that he lured Oichi to him, with the desire to resurrect Nobunaga using the woman's blood. This encounter leads to her Purple Route, where Oichi rushes to get to Azuchi Castle before Tenkai can fully resurrect her brother. Unfortunately, she is too late, and Tenkai is found dead by Nobunaga's hands. The two siblings fight, and at her victory, Oichi drags her brother back down into the depths of hell.
Oichi is prone to calling out the names of those long dead, including her brother, Nōhime, and Ranmaru. If the player fights her as Tsuruhime, she is likely to mistake the young girl as Ranmaru, due to their shared weapon. Strangely, the times she utters Nagamasa's names are in her few moments of clarity.
In Sengoku Basara 2 Oichi is the only character with a unique ending sequence; her image song, "Nemure Hi no Hana", which is sung by Mamiko Noto, her own seiyuu. Her theme would return in the fallowing title Sengoku Basara 3, though as a Piano Arrangement. Due to the game being released outside of japan, as Sengoku Basara: Samurai Heroes, her theme received an official English translation: "Sleep Scarlet Flower".
Oichi makes an appearance in Sengoku BASARA 4 as a playable character. She is unlocked by completing Azai Nagamasa's story. Completing her story will unlock Oda Nobunaga.
And she is reprised by Laura Bailey in the English dub and Mamiko Noto in the Japanese dub.