Ryu Hayabusa - he is main protagonist and playable character in Ninja Gaiden series and debut in original of Ninja Ryukenden in 1988. He is son Jo Hayabusa and nephew of Murai.
Irene Lew - also as Irene Liu and Aileen Von spelling, she is main female character in original of Ninja Ryukenden series and she is an agent of the CIA's Secret Auxiliary Unit, working under the codename "Sea Swallow".
Ayane - she is female character in Dead oder Alive series, debut in DoA1 (PS1 version). She called Hayabusa is "Master Ryu" in order to respect to him and his rank and she is playable in Ninja Gaiden Sigma 2. She is daugther of Raidou and Ayame and half-siblings/cousins of Hayate and Kasumi.
Rachel - she is a fiend hunter from the Holy Vigoor Empire, debut in remake Titel of Ninja Gaiden in 2004 and she is playable in Ninja Gaiden Sigma 2. She is twin sister Alma, who is transformed into Fiend.
Kureha - she was Dragon Shrine Maiden of the Hayabusa Ninja Clan, sworn to protect the Eye of the Dragon, childhood friend of Ryu and elder sister of Momiji. She was killed trying to prevent the theft of the Dark Dragon Blade.
Sonia - she is American CIA agent and lead character in NGII and NGS2.

To be continued...