• KINGDOM HEARTS χ Back Cover (movie)
  These mobile games cover the events of the Keyblade War and set up the lore for the series. They have been remastered as Kingdom Hearts χ Back Cover in the HD collections. Reveals the betrayal among the Foretellers and the mysterious role of the Master of Masters.
  
  
• Kingdom Hearts Birth by Sleep
  A prequel to the original game, focusing on three new characters: Terra, Aqua, and Ventus. Shows how Terra, Aqua, and Ventus's fates become entangled with Xehanort's schemes, leading to Terra's possession and Ventus's heart merging with Sora's.
  
  
• Kingdom Hearts
  The original game where Sora's journey begins. Sora learns about the Keyblade, loses his home, and sets out to find his friends, eventually facing Ansem, Seeker of Darkness.
  
  
• Kingdom Hearts Re:Chain of Memories
  Directly follows the events of the first game and introduces Organization XIII. Sora loses his memories as he navigates Castle Oblivion, while Riku battles his inner darkness.
  
  
• Kingdom Hearts 358/2 Days
  Takes place during the events of Chain of Memories and the beginning of Kingdom Hearts II, focusing on Roxas and Organization XIII. Explores Roxas's daily life in the Organization and his friendships, leading to his eventual betrayal and capture.
  
  
• Kingdom Hearts II
  Continues Sora's journey and concludes many plotlines introduced in the earlier games. Sora battles Organization XIII, reunites with his friends, and defeats Xemnas, the leader of the Organization.
  
  
• KINGDOM HEARTS Re:Coded(movie)
  Takes place after Kingdom Hearts II and deals with a digital version of Jiminy's Journal. Reveals a message in Jiminy's Journal that leads to the discovery of new threats and the return of the real Sora.
  
  
• Kingdom Hearts 3D: Dream Drop Distance
  Follows Sora and Riku as they take their Mark of Mastery exams, setting the stage for Kingdom Hearts III. Sora falls into darkness, while Riku completes the exam and becomes a Keyblade Master, revealing Xehanort's return.
  
  Kingdom Hearts 0.2 Birth by Sleep – A Fragmentary Passage
  A short game that follows Aqua's story after Birth by Sleep and serves as a prologue to Kingdom Hearts III. Aqua battles through the Realm of Darkness, showing her struggle and determination to protect her friends.
  
  
• Kingdom Hearts III
  The latest main entry that brings many storylines to a climax. Sora and his friends confront Xehanort in a final battle, leading to Sora's ultimate sacrifice to save Kairi.
  
  
• Kingdom Hearts: Melody of Memory (NOT IN STEAM YET)
  A rhythm game that recaps the series and provides some new story elements after Kingdom Hearts III. Kairi explores her memories, seeking Sora, and the game hints at the next journey to find him.

• Kingdom Hearts (2002)
  
• Kingdom Hearts Re:Chain of Memories (2004)
  
• Kingdom Hearts II (2005)
  
• Kingdom Hearts 358/2 Days (2009 - was a playable game)
  
• KINGDOM HEARTS Re:Coded (2010 - was a playable game)
  
• Kingdom Hearts Birth by Sleep (2010)
  
• Kingdom Hearts 3D: Dream Drop Distance (2012)
  
• KINGDOM HEARTS χ Back Cover (2017) -
  
• Kingdom Hearts 0.2 Birth by Sleep – A Fragmentary Passage (2017)
  
• Kingdom Hearts III (2019)
  
• Kingdom Hearts: Melody of Memory (2020)

While not essential for understanding the core plot of Kingdom Hearts, the novels and manga enrich the universe by dig into characters backgrounds, side stories, and additional insights. I recommend reading them after completing each corresponding game to enhance your appreciation of the characters and their journeys.