How to unlock all Survivors in Risk of Rain 2

Acrid

Complete the …To Be Left Alone challenge; finish the Void Fields (Hidden Realm found by activating the Null Portal in the Bazaar Between Time).
Under the Bazaar is a hidden cave going to a purple portal to the Void Fields. Beat the nine challenge cells in the Void Fields to unlock Acrid.

Artificer

Complete the Pause. challenge; free the survivor suspended in time for 10 Lunar Coins.
Collect 11 Lunar Coins, then find a Newt Altar. Pay one coin so the blue portal spawns upon charging the teleporter to 100%. Use the blue portal and head to the shop. Pay 10 coins to unlock Artificer.

Captain

Complete the Washed Away challenge; beat the game.

Commando

Unlocked by default.

Engineer

Complete the Engineering Perfection challenge; complete 30 stages. Can be done in multiple runs.

Huntress

Complete the Warrior challenge; complete the 3rd teleporter event without dying.

Loader

Complete the Guidance Offline challenge; defeat the Alloy Worship Unit in Siren’s Call.
Will spawn upon destroying five vulture nests in Siren’s Call in the large open area of the map.

Mercenary

Complete the True Respite challenge; obliterate yourself at the Obelisk.

MUL-T

Complete the Verified challenge; complete the first teleport event 5 times.

REX

Complete the Power Plant challenge; carry the Fuel Array from the (back of the) Escape Pod to the Abyssal Depths.
In the Abyssal Depths, locate the broken REX and interact with it to unlock REX.
!! MUL-T and Acrid are unable to obtain the Fuel Array solo, since they spawn without a pod.