Evolution and Master Tree

Your class evolves twice through quests. The T window shows the ones in progress.

How quests work

You accept them by talking to the NPC. Accepting costs zen, which isn't refunded, and they can't be abandoned.
Turning them in consumes the requested items. Reward points are not lost when you reset.

They're solo: no party

They can't be accepted while in a party. Leave the party before talking to the NPC.
Nothing counts in a party: neither kills nor quest items. If you do the quest as a group, you make no progress. It doesn't pass to the teammate who hit hardest either.
The kill counts for whoever dealt the monster the most damage on their own.

Quest items

They drop from the monsters the quest names, one at a time and not always.
Only you can see them. For other players they don't exist: nobody else can see or pick them up. Your MU Helper does pick them up.
They stop dropping once you have as many as the quest asks for (counting what you carry and what's on the ground for you).
They can't be sold, dropped or put in the vault.

Second class (level 150)

With Priestess Sevina, in Devias (32, 185):

Scroll of the Emperor: bring the scroll. Reward: extra attribute points.
Three Treasures of Mu: bring your class's treasure (Broken Sword, Soul Shard of Wizard or Tear of Elf). Reward: Blade Knight, Soul Master or Muse Elf.

The hero quest (level 220)

With Marlon, in Devias (30, 183): Gain Hero Status, bring the Ring of Honor. Unlocks advanced skills that require it.

Third class (level 380–400)

With Apostle Devin, in Crywolf (48, 228):

Secret of Dark Stone (level 380): three items from powerful monsters. Reward: extra attribute points.
Infiltration of Barracks of Balgass (level 400): enter Barracks and defeat its guardians. Reward: more points and Dragon Knight, Soul Wizard or Noble Elf.

Master level

With the third class at level 400, experience becomes master experience: the bar turns purple and your level shows as "Lv 400+N". Only strong monsters give it.

The master tree

Each master level gives one point. Spend them in the A window, one at a time.
There are three kinds of node: passives, skill upgrades (more damage, shorter cooldown, larger area…) and new skills.
Each node requires points in its branch and, sometimes, another node first.
The Scroll of Oblivion refunds all tree points. Book skills stay.