In the beginning, you only have access to 4 characters. The parenthesis is what they can be closely related to in other games. They are Sieg (warrior), Eir (healer), Dainn (mage), and Tia (thief). It is possible to unlock Lime (nothing else comes close), Dacy (beast master), and Yuki (ice mage). Lime, Dacy, and Yuki must be unlocked through quests or bought through the item mall. There are 2 others that are not released in the English version yet and 1 more character that has not been revealed yet. There is no customization available during character creation. Each of the characters plays quite differently. Characters are gender restricted until the item mall item is released in the US version. The other versions already have an item to change gender.
Sieg is focused on physical attacks and has some knockback attacks and fairly good combos. His attacks are strong, but combos aren't as great as some other characters. Eir has a few combos but has mostly support attacks. Her attacks are weak, but has some offensive spells to make up for it. Dainn has strong attacks, but his combos are few in number. Tia has a strong offense and good combos, but her defense is lower than a warrior like Sieg but still higher than the mages. Slime has a large combo and is heavily attack focused. His defense is decent, but his transformations make him a easy target for attacks with multiple hits. Dacy is a powerful attacker, (Lunia Gold)but combos are low in hits and has low defense and hp. Yuki is similar to Dainn except all ice element. Mages (Dainn and Yuki) have the ability to recharge their own MP instead of having to wait for it. The characters play quite differently from each other. Many of them share similar combos so learning 1 combo can result in using it on 3 characters because of the similarity in their standard attacks. Each character has their ups and downs, but the community tends to favor certain characters over others. As a result, Sieg can have a difficult time getting into some parties.
The game itself is divided into 2 parts. The town square is where you organize parties, obtain quests, set up guilds, do your shopping, and go into the Stage itself. The second part is the Stage where you can fight, level up, and do things with other people. Most Stages allow for a group of 4 people. There are certain dungeon raids that go up to 14 players. The number of players affects the strength of enemies. More players means stronger enemies while less players means weaker enemies. Having more players is always easier than having less players though.