Nicki the Ninja (also known just as Ninja) is an alter ego of Nicki Minaj. She is a hip hop lyrical assassin, which is sometimes very naughty. Lyrically, Ninja appears to "slay" the song with rude and though verses that mostly criticize or diss other people (commonly female rappers). The voice of Ninja is generally quiet and calm but sometimes her voice turns into a fast and tough voice. A good example of the style of Ninja can be heard in the song "Higher Than a Kite". Also, some of the songs where she appears have an Eastern background sound.
It could be assumed that she is featured in the song "Your Love" because of her singing style and the line "When I was a Geisha he was a Samurai", although, lyrically the song is not like Ninja. Also, a Nicki dressed like a ninja can be seen in the video of Your Love which making very likely to be Nicki the Ninja. At the end of the video, another ninja/samurai is seen killing Nicki so it could be assumed that Nicki the Ninja was killed in that video and that would explain why she no longer appears in other songs or videos of Nicki.