Grenadier follows the travels of the extremely busty Rushuna Tendo, an expert Senshi, and the samurai Yajiro Kojima, a mercenary swordsman. A Senshi or "Enlightened" is one who is skilled to some degree in the use of guns.
(taken from Wiki)

Picture	Real Name	Description
	Rushuna Tendo	An extremely busty, blonde senshi who was trained in the positive teachings and kind ideals of Tenshi. A former pre-Juttensen candidate, she is trained in the use of a revolver, specializing in shooting to disable rather than to kill using a form of Gun-Kata as seen in the final battle with Setsuna. Rushuna fights to end strife and chaos. She has excellent tactical analytical skills, being able to discern her opponents' weaknesses in the heat of battle and adjust accordingly to exploit them with the tools at her disposal. She also hugs opponents who are difficult to negotiate with to her chest in order to pacify them. She is 16 years old although she seems older. Rushuna has a weakness for taking hot baths, and she indulges in this pursuit frequently. She also serves as a vehicle for much of the show's fan service, some of which takes surprisingly innovative forms (for instance, storing ammunition in one's cleavage).
	Yajiro Kojima	A swordsman also known as the "Rearguard Tiger" or "Tiger of the Rearguard", who battles in an attempt to bring about world unity. He is called the Tiger by Templar because the more cornered Yajiro becomes, the greater is abilities become. A former rebel, he was working as a mercenary until he met Rushuna. Although Yajiro disdains the use of firearms, he is skilled enough to parry bullets with his sword as well as slice massive cannon shells in two. Yajiro hates guns because he believes that gunners see the weight of a person's life in how hard it is to pull the trigger. However he later states that he does not see Rushuna in the same light as he does other gunners. The route that he, Rushuna, and Mikan take towards Tento's capital by crossing Mt. Charanbo and then White Wizard Lake retraces the same route that he followed when he was with the rebel army that attempted to invade the capital a year and a half ago and is thus memory-laden for him. Rushuna calls him "Yatchan" or "Ya-chan"
	Mikan Kurenai	An orphaned girl who comes from a family of balloon makers. Despite her young age, she is a master balloon artist, being able to construct balloons of all shapes and sizes. After her parents were killed by Tenma Ganzo's senshi, she was rescued by Touka Kurenai, and accepted her offer to join Touka and the other dispossessed girls of her village. Since then she has worked for Touka at the Peach Blossom Tower while nursing dreams of revenge. After Rushuna helps her see beyond revenge, she is unable to kill Ganzo when she has the chance. Mikan then leaves Touka Kurenai and joins Rushuna and Yajiro on their journey, to 'keep an eye on them'.