By blitzing through dungeons, characters will rapidly level up, with levels granting players a point to spend on upgrading one of their skills. Characters have an innate passive and five active abilities, one of which is a sort of ultimate ability. Each of the active ability slots have two options players can pick between to equip and players can even set up a few ability presets to swap between whenever they're out of combat. It’s fairly likely, however, that players will end up choosing a few abilities they like and stick with them, because they have a limited number of upgrade points to spend. For the character I was playing (Noxious), the two choices were usually a support choice vs. a damage choice.
Specifically regarding co-opportunities, Wild Buster makes it easy to jump into the game with a friend or two and start hacking-and-slashing the heck out of stuff. Drops are on a per-player basis and if you get a piece of gear, it will always be for your class. There is a trade system in the game, however, for if you find things you want to trade with you friends or group members. There have only been two small wrinkles I’ve run into in terms of grouping: 1) there doesn’t appear to be a quest sharing system, and 2) I’ve run into one quest (which involved rescuing NPCs in dungeons) that did not allow shared progress for all group members. Other than that, the grouping system works well, with XP being shared to all group members (distance doesn’t appear to be much of a factor).
While the various classes appear to be reasonable self-sufficient, there are many skills that have clearly been created with party play in mind. The character I’m playing, for example, gets a lot of area heals that can really help top off player health, or even be the difference between living and dying during some of the later boss fights. The characters one of my friends plays has a taunt ability that has proved useful in managing enemy hate. So while I wouldn’t say characters are directly reliant on each other, they can often easily support each other to make the group stronger and more efficient.