http://www.baka-tsuki.org/project/im...BD_V01_088.jpg

There are 6 volumes. Kamito believes he has a comrade relationship with all of them though all the girls want to romantic relationship with him. The one exception is maybe Est who Kamito favors over the others. All of the girls have fallen. Est is your loyal spirit. Fianna likes Kamito partly because of their past together in the first tournament and the events of volume 2. Rinslet, Claire, and Ellis are all major tsuderes. Claire fell for Kamito in volume 1. Ellis fell in volume 3. Rinslet..., I'm not really sure.

1. Restia is a dark spirit of the former maou/dark lord. She was also the teacher and then personal sword spirit of Kamito during his early days. Together, they were unstoppable. Fianna is one of the royal princesses. She specializes in large-scale magic, but she lost her ability for years until volume 2. 2. Est is the holy sword of the legendary warrior who defeated the dark lord in the past. That said, the holy sword has a side effect of gradually draining the user's life. When the dark lord was defeated, the wielder of Est also died. That said, even though Est is a sword on par with Restia, her powers are currently nerfed due to certain reasons explained in volume 1 (forgot the reason). She can know only access 1/10th of her original power.
Kamito is a pure sword wielder, though a bloody fine one at that. Based on sword abilities alone, he is among the elite. 3 years ago, he absolutely dominated the competition as his alter ego. He was so dominant that all the girls in the present day still admire him (though they believe Ren is a her). People like Claire and Ellis still look up to Ren as the gold standard. That said, because Est is nerfed, Kamito is also nerfed. Further, when the story started, Kamito was a bit rusty. It's only until volume 6 that he's gradually regaining his old skills.

I guess if you saw/read Legend of the Legendary Heroes, Kamito is similar to that MC in their background. Kamito was raised by his countries' secret assassination organization. At the time, it was still a secret, but in the current timeline, it's publicly known. Of course, that organization no longer exists either because the whole place burned down.

That organization, which has a name that I forgot, picked up kids and raised them to be assassins. They would often pit these kids against each other and let them fight to the death. Kamito was the best of the best and killed everyone who crossed swords with him. After being recognized as the best by his trainers, he was given to Rentia. From then on, Rentia started to train Kamito. At a certain point, Rentia became Kamito's personal sword and they started a partnership that would dominate the first tournament.

why did he quit after that long a time? Is that why he knows the principal??

I don't remember the details, but from what I remember, he left after the organization was burning down around him. He went and retrieved Rentia and later went into the real world. His background was primarily explored in volume 4 when a fellow assassin who used to call him "brother" returned as part of the evil organization that the current "Ren" is part of.

His past with the principal isn't that closely explored. That principal is actually really, really powerful and one of the few that could potentially be stronger than the old Kamito. She had the position as one of the 10 royal sword dancers (and even among them, the best) in the country until she voluntarily retired. She even taught Kamito some of his ace moves. For example, the ace move he revealed in volume 6 which launched 16 consecutive strikes at his/her opponent, was one such move.