Tiers

Tier 0 - Wisp
Start new game as Wisp by typing in chat "-new".

Note: Secret Class

Tier 1 - Angel/Devil/Human
Once hitting level 30 with wisp, go to the spawn area (fastest way by typing "-suicide". Then walk into "Helper Luchifare" which is the Top Left NPC at the spawn.

Kill either Angel or Devil for their corresponding spirit with a small chance of getting Human spirit as drop. Put the spirit in your inventory and use "Crossroads of Reincarnation" skill as a wisp to become that class of which spirit you had in your inventory.

Note: Hidden Class

Tier 2 - Big pool of choices
If you decided to become either human/devil/angel, then upon hitting level 200 you can change into various classes. To do that you need to kill boss at the "Awakening" portal, which is bottom right from the spawn. Once you kill him, he can drop various classes (he does respawn). Once you pick up the class in your inventory use a skill "Awakening"
 * Nightmare (AGI/INT dps)
 * Slaughter (AGI dps)
 * Revenge (STR dps/tank)
 * Disgust (STR tank)
 * Lust (INT support/summoner)
 * Death (str/int dps/summoner)
 * Blessing (int healer)
 * Thirst (str/agi dps/tank)

Tier 3 - Advancement of T2
To become Tier 3 you need the following. Then you need 3 item drops Once you have your 3 required items. Put them in your inventory and use "-create" command. Then with newly created item "Lucifer's Approval", go to the "Fallen Underground",  lvl 500 area, town. And move directly up/north towards the church. Once you enter church, move directly north again to the black circle. This will make your character to Tier 3
 * LvL 1000
 * Base stat (white) at least 10,000
 * Samuel's Blessing, dropped by "Prophet Samuel" in "Sanctuary of Prophet's Rituals"
 * Fallen Angel's Blessing, dropped by "Fallen Angel" in the same area
 * Lucifer's Blessing, dropped by Thousand Faces "Luchifare" in the "Luchifare's Trial area"

Hidden Tier 3 - Alternative path of human/devil/angel
To become hidden tier 3 you need the following:
 * LvL 2000 angel/devil/human
 * Complete Trial

Trial of Alacrity (Schneider/Cloud)
With lvl 2000 human, go kill "Bandit King Marble" in "Middle Earth" (lvl 1 area). This will teleport you to Trial arena, where you will have to hit Marble 100 times before he hits you 100 times.

Trial Of Decay (Sitael)
With LvL 2000 angel, take Azreal spirit at "Place where air of death Hangs" (lvl 700 area) and go to Sitael's summoning circle at "Area restricted by Sitael". This will trigger challenge, where you have to kill "Azrael". You gain more DMG the lower HP you have.

Trial Of Darkness (Sephiroth)
With LvL 2000 devil, take Arthas spirit at "Fallen King Palace" lvl1 area second zone. With Arthas spirit move east after going downstairs into the black area. This will trigger challenge, where you have to kill Schneider.

Tier 4
Global Requirements of becoming T4: Class Specific Requirements:
 * Base stat (white) 250,000
 * Paimon's Grieving Soul (soul)
 * Armor Of Misery (armor)
 * Prideful Leather Glove of Assassination (weapon)
 * Disgust (Use "Giant Explosion" R skill)
 * Revenge (Go to "7th Corps Training Center")
 * Slaughter (Kill 100 enemies)
 * Lust (Hit Abel - Human Race's First Murder Victim (Cain Flag Boss))
 * Death (In LvL 1000 area of "Hell", kill 5 of "Fragment Of Damien", which spawns boss Damien, whom you have to kill personally.
 * Thirst (Use "Blood Explosion" Q skill)
 * Nightmare (Kill 100 enemies)
 * Blessing (Enter church in "Fallen Underground" and walk to "Prophet Samuel"

Hidden Tier 4

 * Sephiroth (Walk into black area once again)
 * Schneider (In "Fallen King Palace" lvl1 area second zone. Go in black area, which is east after going downstairs)
 * Sitael (Go to Azrael Summon Circle in "Lesser Demon Colony". Note: Flag boss must not be activated.

Secret Tier 3.5 & 4 (Wrath/Satan)
To become Wrath T3.5 you need 200k base stat Wisp and go to "Altar of Sin" (Paimon boss fight) area and walk to the Altar of Wrath (top left one).

To Become Satan (T4) you need to obtain Class Weapon then go to "Altar Of Sin" and use your "Authority of the Chosen" F skill.