Well, I have an idea for Love Ronin Solid 2 that I'm going to run past you guys.
There are four Ronins, and each has a different codename based on the Four Heavenly Kings/Swords/Elements.
Keitaro represents the Tiger, which is Land. Hence, Solid Ronin.
Kanako represents the Phoenix for how much she travels, and also the Air. So, she is Vapor Ronin.
Mutsumi represents the Turtle for obvious reasons, and thus she is of the Sea. Her codename is Liquid Ronin.
And there would be a fourth Ronin, representing the Dragon and he/she would be of the Aether. Their codename would be Plasma Ronin.
Each of these Ronins is connected to the Hinata Sou through promises, and therefore each is bound in one way or another to that power.