First, if you have ever paused the game at the character select screen, you might have noticed an option in the menu named "Hidden Char. and Extreme mode" that is an in-game guide that includes instructions of how to unlock different characters and a different mode in the game. You can navigate to the next page by pressing Weak slash (WS) and Strong slash (SS) together, or the button you assigned as the combination of both buttons. It has the same contents as this guide.

So, what you want to do is:

1.- Put your character select box over Kaede (the blond swordman at the top left)
2.- Carefully press the Kick (K) button 10 times, then the Strong Slash (SS) button 5 times, then the Kick(K) button again 4 times. You got a reasonable amount of time to enter the sequence completely, you don't need to enter it super quick.
3.- You should hear Kouryu say "Mokko chuu" once you finish entering the sequence.
4.- At first glance, nothing seems to have changed, but all you have to do is select Kaede with the Weak Slash Button (WS) and Suddenly you will see Kaede's picture switch for Kouryu.
5.- That is it, now you can play as the most OP character in the game.

You can also play as Kaede in his "not sayan" mode from the original Last Blade, He plays similar to his powered up version but has different animation and hitboxes.

So to play wth him:

1.- Highligh Kaede in the character selection (top left)
2.- Press Kick (K) 9 times, Strong Slash (SS) 1 time, then Kick (K) 4 times, as before, you have enough legway to press the sequence without stressing out.
3.- You should hear Kaede's voice say something if you did it correctly, simply press Weak Slash (WS) to select him and you will see his portrait change to his original form.

This code is supposedly to play as Naoe Kotetsu (Shigens daughter) but in reallity is just a code that unlocks a different intro for Shigen. Its a nice joke I guess :P

1.- Highlight Shigen (bottom right character)
2.- Press Kick (K) 5 times, Strong Slash (SS) 10 times then Kick (K) 2 times.
3.- You should hear Naoe speak if done correctly. Simply select Shigen with Weak Slash (WS)
4.- Once in game you should see a different intro to Shigen were he takes Naoe's place.

This code makes you play as a copy of the character you are going to fight, think of it as a Doppelgänger mode.

1.- Highlight Akari (left row, third character from top to bottom)
2.- Press Kick (K) 8 times, Strong Slash(SS) 9 times then Kick (K) 5 times
3.- You should hear Akari's voice if done correctly simply press Weak Slash (WS) to select her and her portrait will change.

Besides Power and Speed modes for the characters there is a secret EX mode.

NOTE: This one has to be input quite faster than the other codes!

According to the guide you want to:
1.- Select your character
2.- Once in the mode selection screen Highlight "Speed mode" and press Kick(K) 6 times, Highlight "Power mode" and press Strong Slash (SS) 3 times then Highlight "Speed mode" again and press Kick (K) 4 times. Need to be quick!
3.- If done correctly your character will automatically switch to EX mode. Simply press Weak Slash (WS) to use.