Kaori is like Ryuuya from Timeranger. Just because Kaori was only for four episodes doesn't mean she should be considered an extra hero. She is simply just another person using the Shinken Red powers. She's probably best looked at as a secondary Shinken Red.
Hidden Masta, you made a good point on how Sentai teams operate in their own ways. There is no true classification of a sixth ranger or an extra ranger, or even maybe their team pet. It just depends on what the writers want them to be used for.
[ hide] Here is how I look at a three people team (four in JAKQ). So far, all of the three people teams have been red, blue, and yellow. I like to think of yellow being the yellow hero, but at the same time sort of filling in for green.
JAKQ vs Goranger is what gave me this view. Aka Ranger and Spade Ace worked with each other, Mido Ranger and Clover King worked with each other, Ao Ranger and Dia Jack worked with each other, Momo Ranger and Heart Queen worked with each other, and Big One's role was almost like Deka Masters in Dekaranger vs Abaranger. Now who did Ki Ranger worked with? He was grouped in with Mido Ranger when working with Clover King.
As for blue, I like to think of blue as being the blue hero while filling in for pink regardless of gender. Now I will get to this later.
As for Liveman, Hurricanger, and Go-Onger here is more in depth of how I see them. Yes, I look at them as being extras. In Liveman, for the two other Liveman who show up later, I see Green Sai as Blue Dolphin's counterpart, and Black Bison as Yellow Lion's counterpart. Blue Dolphin does co pilots Green Sai's mecha, while Yellow Lion co pilots Black Bison's mecha. It's the other way around when using Live Robo. Since Hurricanger is a homage to Liveman, Kuwagata Raiger is clearly meant to be like Green Sai and Kabuto Raiger is clearly meant to be like Black Bison. Therefore making Kabuto Raiger Hurricane Yellow's counterpart (when not bringing up Hurricane Red) and Kuwagata Raiger is Hurricane Blue's counterpart. It's the same thing with Go-Onger, mainly because Go-On Blue suit is ranked the highest of the main three not bringing up Go-On Red, and Go-On Green is ranked the highest of the secondary two.
In Hurricanger vs Gaoranger, Gao Red worked with Hurricane Red and Gao Silver worked with Shurikenger. Now based on my way of thinking, Gao Blue and Gao White would work with Hurricane Blue while Gao Yellow and Gao Black worked with Hurricane Yellow. Well that's not really the case. Gao White worked with Hurricane Blue while Gao Blue worked with her Goraiger counterpart Kuwagata Raiger (Since crossovers seem to group people together based on sex), and Gao Yellow worked with Hurricane Yellow while Gao Black worked with his Goraiger counterpart Kabuto Raiger.
Gao Blue worked with Kuwagata Raiger while Gao White worked with Gao Blue. Since Gokaiger does it right and not let sex get in the way, Gokai Blue is Hurricane Blue while Gokai Pink is Kuwagata Raiger. So this explains why I would like to think of blue being the blue hero of the team while filling in for pink.
It's hard to put Big One, Abare Black, Geki Violet and Geki Chopper in this category. In JAKQ and Abaranger, the red heroes of both teams are the older siblings, while Big One and Abare Black are the daddies of those teams. Geki Violet uses forbidden powers (which is something you'd expect of sixth heroes), and Geki Chopper powers is seperate from the teams more than Geki Violet.
I admit that Gokaiger does this only with Hurricanger, with Liveman and Go-Onger they just have them do it by color. This is why I don't 100% trust how Gokaiger classes heroes. I trust it, but not 100%. [/ hide]