Here's a full list of every unlockable class/character. There's unlock information located in this thread as well http://www.gamefaqs.com/boards/688762-knights-of-pen-and-paper/64499734 Hope this helps.
Here's all the "secret" classes you can obtain in the game.
Barbarian The Barbarian is pretty simple. Just use Berserk on him and watch him tear monsters apart and keep himself topped off with Blood Seeker.
Shaman Hurricane will tear enemies up real quick. Static Field can increase Magic of the target you use it on, which will make Hurricane more potent and gives you more MP to play with. I felt this one was a good upgrade over the Mage class.
Abilities: Hurricane Static Field Thunderstorm Vine Trap
Necromancer ...what's the point of this class? Really? Can anyone elaborate on this? Just one look at the abilities made me think "WTF are they thinking?!".
Abilities: Death and Decay Energy Drain (Passive) Soul Pact Summon Minion
There's also 5 players you can have join your group. You can obtain them all in one area, actually.
Ronaldo Drinker (Passive) 100% extra potion effect and 5% of life steal.
May be a good candidate for the Paladin and Knight classes, but not the best. He can fill any role just fine.
Betu Arretado (Passive) +3 bonus to attack and 50% less on player resurrect.
Excellent for any melee DPS class in the game, such as Barbarian, Warrior, Rogue and Hunter.
Gui Everyone's Ally (Passive) Reduce threat by -5 (100%) and 5% of mana steal.
Great character to use for the Druid, Cleric and Bard classes because of the reduced threat which makes him less likely to be attacked which is very nice for a healer class.
Hugo Limit Break (Passive) Extra 5 (100%) threat and 10% to critical.
Best character to use for the Paladin class, hands down. He can easily get the highest Threat in the game which makes him an excellent tank.
Saulo Leadership (Passive) +5 to initiative and 30% extra XP.
Basically an upgrade to the Little Brother character, except that, uh... the 30% extra XP doesn't actually seem to work in the game. Right now you can use him or Little Brother, whatever you feel like up to using.