While not as good as Dawn of Sorrow, this is a great game that is definitely worth playing.
You'll probably stick with Jonathan, just pressing the R-button every once in a while to have Charlotte use a spell before disappearing again. The X-button, with which you swap characters, won't be used much. The only times you'll want to use Charlotte are in the very few puzzles where you actually need her to proceed. There is no reason at all to use Charlotte - her offensive and defensive powers are inferior to Jonathan's, and she needs to chant before using a spell, leaving her vulnerable. The reason there are two characters in this game is the Julius mode in the first DS Castlevania, Dawn of Sorrow, where you had three different characters to swap between.
However, in Julius mode the three characters were balanced, each with their own pros and cons. Charlotte, however, is completely useless. She has some good spells, but as I said, pressing the R-button will cause her to appear, use her spell, and then leave. Did we really need another character for that? Why couldn't they simply have Jonathan use the spells instead? Whatever...
Jonathan has his own counterpart to Charlotte's spells, including a handgrenade, a throwing-star, the subweapons from the old CV titles(cross, bible, holy water, knife) and much more. In a particular room in the castle, there is a ghost who gives you quests. He gives you some pretty sweet rewards when you complete them, like skills, spells and weapons.
And yeah, then there's the story. The story is, as in most other CVs, not that important. A new vampire called Brauner, accompanied by his two daughters Stella and Loretta, has taken control of Dracula's castle. Jonathan, the current owner of the Vampire Killer whip, sets out to defeat him, with the aid of his friend Charlotte.
The setting is, as usual, in Dracula's castle. Most of the time is spent out of the castle though. To stop Brauner, you need to enter his magical paintings and defeat the bosses in them. The paintings will take you to eight different places(the last four are pretty much the same as the first four, just recolored and with different enemies), ranging from an Egyptian pyramid to a hotel in a swamp. While these areas are nice, IMO they should have focused more on the castle.
The graphics are beautiful 2D backgrounds and sprites. While not as visually appealing as the previous DS CV, the graphics are still outstanding. The horrible anime-style character design from DoS is still present, but luckily we don't have to see those god-awful faces that much. The enemies look good, and the boss fights are terrific. The music is, as always, great.
There are a few unlockable modes, like Sisters mode, where you play as Stella & Loretta, Richter mode, where you control Richter and a child version of Maria, and Old Axe Armor mode, where you use an Old Axe Armor enemy to chop up Drac's minions. Richter mode is nice, but none of these extra modes can compare to Julius mode in DoS.
Another new thing is the level caps in hard mode. You can choose to set the cap at level 50, 25(?) and 1. If you think this game's a piece of cake, just wait until you try playing it through with level 1 cap and no equipment.
All in all, this is a great game, with a few flaws. If you don't own Dawn of Sorrow, buy that instead, as it's a better game IMO. If you have it and enjoyed it, you'll definitely enjoy this too.