Classes are sets of character skills and stats in Crystal Story II.
In Crystal Story there are six classes: Slayer, Thief, Elementalist, Guardian, Healer, and Trickster. Classes are further broken up into modular tiers, which characters progress through one at a time. Each character first completes a Tier I module for one class, then moves on to another Tier I, then two Tier II classes, continuing in this fashion through III-V with two classes at each tier. After the character has attained two Tier V classes, another Tier I class may be taken, then another Tier II class, and so on.
Note that many different combinations are possible and characters are not tied to a class; for example, nothing prevents a character with Thief I and Trickster I from taking Healer II as a Tier II class. Such a character will not retroactively recieve Spells from Healer I, but will recieve Healer II spells directly.
The first three characters in the game have their initial classes preselected; otherwise, the player is free to choose any combination of classes.
Each class behaves slightly differently for each character, granting slightly different stats and sometimes different spells.
D gains 4 SPD, 4 HP, 5 ATK, 1 MP, and 2 MAG nodes, while Lina gains 7 SPD, 3 HP, 4 ATK, 1 MP, and 1 MAG node. Clearly Lina got a lot more SPD while D got more HP, ATK, and MAG.
This is not the only difference: the skills D obtains are Throw Burst 2, Quicken, and Sprint, while Lina learns Throw Burst 2, Quicken, and Pilfer. D will need to take Thief IV if he wishes to learn Pilfer!
Some Tier V spells are unique to a certain combination of character and class; planning is necessary if one wishes to take advantage of these spells.
Each tier Class contains between 22 and 27 nodes. Each node costs SP to learn. The SP cost varies between the base SP cost up to 1.2 base SP cost as the character finishes learning that class in that tier. Each successive class costs more SP. After learning 2 classes in all 5 tiers, a character may add a 3rd class to each tier, then a 4th, 5th, and 6th. The table below gives the base SP cost for nodes by tier.