I know what you're thinking. You're thinking "Oh great and omnicient Hiryuu, how were you able to figure out this experience stuff?"
The answer may surprise you. I kept careful track of my experience I got on a calculator, and when I got really close to leveling up, I fought Evil Dolls (which give only 1 experience point) until I went up to the next level. This gave me an accurate experience points to level up, and it only took me forever to do! ^_^
I know you know that you're not able to see your experience level in the actual game, and my work was probably all in vain, but if nothing else, this'll let you estimate how long it'll take you to level up. And yes, Level 20 is as high as it goes.

* "Experience needed from last level" how many points do you need to level up from the last level (basically if you want the experience needed from last level for level y, and you used to be on level x, it would be the total experience for level y minus total experience for level x).
** "Maximum Pray points usable" refers to how many pray points you can use at one time
*** Also, remember, all these stats are assuming you have no weapons equipped.
**** One last thing, the HP values are only true for the female characters (Emi, Asuka, and Akiko). Kazuo's HP will always be 50 more than the given, and Taro's will be 20 more.
***** Ok, this is the absolute last one, I promise. The characters' defense is constant throughout the entire game. It can't be raised. I believe that Taro has the highest defense, followed by Kazuo, then Emi, then Asuka, then Akiko.