Optional Rule: Level 0 Characters
This rule is intended for games in which you want to have characters with truly humble beginnings working towards heroism while still keeping a semblance of what they will become. You may also use this rule to balance one or more PCs having a significant edge at the earlier levels due to their choice of race or other such feature, but you still want to include them without penalizing them. Regardless, this rule is meant to grant you the freedom to explore new possibilities and provide a different gaming experience.
Creating a Level 0 Character
To create a character using this rule, the players go through character creation as normal choosing their background, race, and class, however when it comes to this last point the character gains only the hit points, proficiencies, and starting equipment of their chosen class, but none of its 1st-level features. For example, a fighter created using this rule would have 10 starting hit points (as well as a 1d10 hit die to spend during rests) and gain proficiency with all armors, shields, as well as simple and martial weapons, but would not gain the Fighting Style or Second Wind features.
If your chosen class would gain the Spellcasting or Pact Magic features at 1st level, your character would learn and be able to cast the same number of cantrips they would have available at 1st-level using the spellcasting ability listed in those features, but they would not gain access to any of the class's spell slots or spells.
Advancing Beyond Level 0
A character created in this manner gains experience and advances in levels at the same rate as a normal player character, the only difference being the lack of features one using this rule would have. Once a 0 level character gains a total of 100 experience points, they officially advance to 1st-level and gain all of their class's features and in the case of a class with the Spellcasting or Pact Magic feature they would also gain access to the class's spell lists and spell slots.
In the case you are using milestone leveling for your campaign, it is advised that you set a different earlier milestone to level for a 0 level character if they are in a party with regular characters.
Exceptions
If your chosen character class would gain the Unarmored Defense class feature at 1st-level, a level 0 character using that class would also gain the feature.
If your chosen character class is the Artificer, then you both gain the Magical Tinkering feature and retain the need to use tools as spellcasting focus in order to cast your spells.
If your chosen class is the Cleric, you gain any proficiencies your chosen domain would grant at 1st-level.
If your chosen class is the Monk, then your character may use the martial arts die the class gains at 1st-level for their unarmed strikes and monk weapons.
Art Credit:
"Aleena" by Larry Elmore
Sample Character

Aleena, the Cleric
Medium, Lawful Good
- Armor Class 18 (chain mail & shield)
- Hit Points 10 (8 + CON mod)
- Speed 30 feet
STR DEX CON INT WIS CHA Str 14 (+2) Dex 10 (+0) Con 14 (+2) Int 11 (+0) Wis 16 (+3) Cha 14 (+2)
- Saving Throws Wisdom, Charisma
- Skills Insight, Perception, Persuasion, Religion
- Languages Common, Dwarven, Elven, Halfling
Background
Acolyte
Equipment
- Mace, chainmail, dagger, priest's pack, shield
- holy symbol
Features
Bonus Proficiencies: Heavy Armor (Life Domain)
Cantrips
- Guidance
- Sacred Flame