How to get Ruby and pass Floor 9
The first time you enter this floor you'll get a funny little cutscene involving two undead and a canon. As you probably already guessed upon making it to floor 9 you'll need to get another ally in order to progress forward. This ally has arguably the worst doors to bypass in the game.
Xiao has the annoying cutscene and Gordo does too but this character you have to go into your inventory and change the element of your weapon to the same one as the crystal (which that part alone confuses most everyone) and then shoot the crystal with that element to open the door. It's such a pain in the arse!
Luckily getting Ruby isn't even half as hard as opening her doors are! In order to acquire Ruby you'll want to complete the King's Hideout in Queens. After you complete his house you'll have a short cutscene where he gives you a lamp but like a dumbass you accidentally knock it over and out comes Ruby. Ruby is your first ranged magical caster. In my opinion Ruby is a bit worse than Xiao is so if you're looking for a good ranged character, just stick with Xiao.
I mention it in a few other places throughout my Dark Cloud guide but I figured here is probably the best place to shed some light on how much Ruby's doors and opening them totally blows. Unlike most other characters who you just switch to, hit a button and have a brief cutscene Ruby you have to switch to, change her weapon's element, use first person view and then fire at the crystal. Also, some elements look alike so it can make it even more of a pain in the ass.
Point of bringing it up is I have a guide about opening Ruby's Doors you may want to take a look at if you're having troubles.