Games like Milli & Greg
Games like Milli & Greg include Celeste, Super Meat Boy, SANABI, Geometry Dash, and Momodora: Reverie Under The Moonlight. We matched them by the tags players give Milli & Greg on Steam (Female Protagonist, Precision Platformer, Difficult, 2D Platformer, and Platformer): closest matches first, and among equally close ones the better-known games.

Celeste
Shared: Precision Platformer · Difficult · Female Protagonist

Super Meat Boy
Shared: Precision Platformer · Difficult · Platformer

SANABI
Shared: 2D Platformer · Pixel Graphics · Action

Geometry Dash
Shared: Difficult · Precision Platformer · Platformer

Momodora: Reverie Under The Moonlight
Shared: 2D Platformer · Female Protagonist · Side Scroller

VVVVVV
Shared: Precision Platformer · 2D Platformer · Platformer

Hollow Knight: Silksong
Shared: Difficult · Female Protagonist · Platformer

Jump King
Shared: Precision Platformer · Difficult · Platformer

Downwell
Shared: Precision Platformer · Pixel Graphics · Platformer

Touhou Luna Nights
Shared: Difficult · Female Protagonist · Side Scroller

Planet of Lana
Shared: Female Protagonist · 2D Platformer · Action

Mina the Hollower
Shared: Female Protagonist · 2D Platformer · Pixel Graphics

Skul: The Hero Slayer
Shared: Pixel Graphics · Action · Difficult

Hollow Knight
Shared: Difficult · Platformer · 2D

Katana ZERO
Shared: Pixel Graphics · Platformer · 2D Platformer

Shovel Knight: Treasure Trove
Shared: Platformer · Pixel Graphics · Difficult

Pepper Grinder
Shared: Difficult · 2D Platformer · Female Protagonist

The End Is Nigh
Shared: Precision Platformer · Difficult · Platformer

They Bleed Pixels
Shared: Difficult · Female Protagonist · Platformer

Dungeon Escape
Shared: Precision Platformer · Difficult · 2D Platformer

Gravity Circuit
Shared: Platformer · 2D Platformer · Pixel Graphics

Pizza Tower
Shared: 2D Platformer · Platformer · Pixel Graphics

NINJA GAIDEN: Ragebound
Shared: 2D Platformer · Platformer · Action

Prince of Persia The Lost Crown
Shared: 2D Platformer · Precision Platformer · Side Scroller