Probably the only reason why you've decided to play this game.
You farm gifts via Exploration Quests and give enough to reach the max of 100 Intimacy with a character to unlock a character's still. Other ways to receive gifts are through Login Rewards and Event Quests.
Note: Mob characters share intimacy with each other. Alternate variants also share intimacy with their original counterparts.
For data on how often a character may have appeared on stills, you can refer to Character Appearance.
Story quest titles marked with ★ indicates an NSFW still is present in the story. These are available for free, and can be opted out through choosing dialogue options or through exiting if left with no choice.