Games like Heartless
Games like Heartless include Thank Goodness You're Here!, Hollow Knight, Sonic Origins, Pikuniku, and Celeste. We matched them by the tags players give Heartless on Steam (Casual, Adventure, Platformer, 2D Platformer, and Mystery Dungeon): closest matches first, and among equally close ones the better-known games.

Thank Goodness You're Here!
Shared: Adventure · Casual · 2D Platformer

Hollow Knight
Shared: Platformer · 2D · Adventure

Sonic Origins
Shared: Adventure · 2D Platformer · Platformer

Pikuniku
Shared: Adventure · Casual · Cute

Celeste
Shared: Platformer · 2D Platformer · Adventure

Laughing to Die
Shared: Casual · Adventure · Platformer

Webbed
Shared: 2D Platformer · Cute · Platformer

GRIS
Shared: Adventure · Platformer · Hand-drawn

The Purring Quest
Shared: Platformer · Cute · Adventure

Castle of Illusion
Shared: Adventure · Platformer · Casual

Neva
Shared: Adventure · Platformer · Emotional

Florence
Shared: Casual · Adventure · Cute

Scribblenauts Unlimited
Shared: Casual · Adventure · 2D

Hollow Knight: Silksong
Shared: Platformer · 2D · Adventure

SANABI
Shared: Adventure · 2D Platformer · 2D

Sonic Superstars
Shared: Platformer · Adventure · Casual

BEEP
Shared: Adventure · Platformer · 2D

Chained Together
Shared: Adventure · Casual · Platformer

Slime Rancher
Shared: Adventure · Cute · Casual

Yoku's Island Express
Shared: Adventure · Platformer · Cute

Ori and the Will of the Wisps
Shared: Platformer · Adventure · Cute

DuckTales: Remastered
Shared: Adventure · Platformer · Casual

Super Fancy Pants Adventure
Shared: Adventure · Casual · Platformer

FEZ
Shared: Platformer · Adventure · Casual