Growth is also a theme. Tanjiro and Tomioka grow both as individuals and in their relationship. Tanjiro might learn from Tomioka's skills, and Tomioka might be inspired by Tanjiro's determination. This is shown in many fanfics where they train together or face difficult situations side by side.
One common theme is friendship. They often start as comrades in the Demon Slayer Corps and develop a deep friendship. For example, they support each other in battles.
A practical answer is Chapter 2, because it turns the idea into a concrete scene. The chapter opens with On the top floor of the Financial Building, a gathering of dark clouds..., adds pressure through In my opinion, Mother should kick out this troublemaker who's a stranger., and ends on Jiang.
Chapter 637, "Does President Shang Know That His Own Family Is Already Bankrupt?", is the clearest early answer because it turns Shang Shaojing's situation into a concrete plot event.
Chapter 840, "How did you meet Miss Jiang?", is the clearest early answer because it turns Gu Qiang's situation into a concrete plot event. The chapter turns that into something concrete by moving from Jiang Li seemed surprised that she would suddenly speak, paused for a...
Chapter 578, "Eight Men Sit in Your Lap and You Don't Flinch at All?", is the clearest early answer because it turns Let's situation into a concrete plot event. The chapter gives readers a real answer by opening on This is ten thousand points we're talking about!" "Didn't you read...
Chapter 502, "Let Shaobai Follow Along", is the clearest early answer because it turns Shen Ruobing's situation into a concrete plot event.
Chapter 488, "You Must Be the Captain Then!", is the clearest early answer because it turns Nina's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Lose face?" Nina's voice rose sharply, "We're already at a dead end,...
Chapter 587, "Has He Also Become a Part of the Play?", is the clearest early answer because it turns Crown Prince's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Zhao Lang said "thank you," but his eyes reluctantly glanced at Jiang...
Chapter 845, "Turns Out He is the Body Double!", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. The chapter turns that into something concrete by moving from During these two hours of waiting for the results, the restaurant was...
Chapter 841, "Isn't this Daniu?", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event.
Chapter 636, "Beg Me", is the clearest early answer because it turns Crown Prince's situation into a concrete plot event.