Affection mechanics

Affection Mechanics is a hidden gameplay element that governs certain scenes during Final Fantasy X. The player can govern Tidus's actions with other party members which ultimately decides which party member talks to Tidus on certain scenes as well as who throws the blitzball during Tidus's Overdrive Blitz Ace.

How each character is chosen is based upon a hidden "affection" value in the game's code. All of the main characters (except Tidus) have an affection value. At the beginning of the game, all characters' affection values start at zero.

Decisions
Throughout the game there are times where Tidus's actions can increase or decrease other party members' affection value.

During Battle

 * Whenever Tidus heals a party member, their affection value increases by one.
 * Whenever Tidus intercepts an enemy attack intended for another party member using either Guard or Sentinel, their affection value increases by one.
 * Whenever Tidus attacks another party member, their affection value decreases by one.

On the Field
During times where other party members are spread out across the area, the first party member Tidus talks to will increase their affection value. All other subsequent conversations with other party members in the same area will have no effect on their affection values. The amount at which the affection value increase will vary by party member, as shown in the following table:-

In Besaid Village (Night)

 * After speaking with Yuna for the first time, Tidus is asked by Wakka "She's cute, ya?". Replying "Yeah!" will increase Yuna's affection value by eight.

On board the S.S. Liki

 * Speaking to Wakka before approaching the crowd of people around Yuna at the bow of the ship will increase Wakka's affection value by four.

On board the S.S. Winno

 * Watching the conversation between Yuna and Wantz before eavesdropping on Lulu and Wakka will increase Yuna's affection value by four.
 * Speaking to Yuna after she converses with the Luca Goers, but before Tidus attempts the Jecht Shot will also boost her affection value by four.
 * Eavesdropping on Lulu and Wakka a total of four times will increase both their affection values by four. This must be done before attempting the Jecht Shot or speaking to Yuna after she conversed with the Luca Goers.
 * Straight after eavesdropping on Wakka and Lulu a fourth time, speaking to Wakka will further boost his affection value by four.

In Guadosalam

 * If Tidus talks with Lulu after Yuna enters Seymour's mansion, when talking to Lulu a third time Lulu will say "I shouldn't have to say this, but don't fall in love with her [Yuna]". Replying "Too late" will increase Yuna's affection value by eight. Replying "You're more my type, Lulu" will increase Lulu's affection value by eight.
 * If Tidus talks with Rikku after Yuna enters Seymour's mansion, she will ask "It's your big chance, huh?". Replying "I guess you're right" will increase Yuna's affection value by eight. Replying "I'd rather have you, Rikku" will increase Rikku's affection value by eight.
 * Watching Kimahri going out of the item shop will increase his affection value by six.
 * When the player returns to Guadosalam later on in the game, but before defeating Ginnem's Yojimbo in the Cavern of the Stolen Fayth, speaking with Lulu in the Farplane will boost her affection value by eight.
 * After defeating Ginnem's Yojimbo in the Cavern of the Stolen Fayth, speaking to Lulu in the Farplane will further increase her affection value by eight.

On the Thunder Plains

 * Outside the Travel Agency, if Tidus enters the Travel Agency before Auron concedes to Rikku, Rikku's affection value increases by eight.
 * Speaking to Rikku inside the travel agency before meeting Yuna in her room, increases Rikku's affection value by eight.
 * After speaking to Rikku inside the travel agency, Rin enters the foyer. If Tidus speaks with him, Rin will ask "Would that be Sir Auron by any chance?". Replying "Nope, wrong guy" will increase Auron's affection value by eight.

Underneath Lake Macalania

 * When under the lake ice, before speaking to Rikku, if Tidus speaks to Lulu she will ask "We're under the lake ice, aren't we?". Replying "Maybe" will increase Lulu's affection value by eight.
 * After speaking to Rikku, but before speaking to Auron, if Tidus talks to Wakka he will ask "Say, you feel something weird in the air?". Replying "Yeah" will increase Wakka's affection level by eight.