The Engineer

Unlocking
1. Obtain the Shovel. This can be found in the Forest.

2. Defeat the Gelatinous Cube, found in Sewers 3. The shovel is required to spawn the boss.

3. The Gelatinous Cube will drop the Ice Key. The shovel is no longer required, so you are now free to pick up a better weapon.

4. Defeat the Gelatinice Cube, found in Crypt 3. This enemy is locked behind a door the Ice Key is needed to open.

5. Talk to the Engineer, who was trapped inside the Gelatinice Cube. Congratulations, the Engineer is unlocked.