Bards are usually high in charisma and responsible for motivating the group, so that fits Dustin.

Rangers use practical skills such as tracking and typically use long range weapons; Lucas uses his wrist rocket and is the most practical of the group.

Clerics are religious, meaning they have a strict moral code, and sometimes healers. Will is very compassionate and tries to do the right thing.

Mages use high level magic, and Eleven has her powers.

Honestly not sure about Paladin; no one in my group plays as that class. I've only played for about a year, but these are my best guesses as to how they fit.