Side Memories is a feature from the original Mobage that showcases a special story about a particular Idol. Chapters are unlocked by earning Side Memories points through completing Event work. A special limited Item called "Heart Slips" (心の付箋) can also be earned from Event Gacha, which is used in a special kind of Business to earn more Side Memories points.
As Side Memories is still a relatively new feature, special Gachas will be held when an Idol's Side Memories story is released. The special Gacha will have the selected Idol's entire card history available for the Gacha, both Rs and SRs.
After unlocking all of an Idol's story chapters, a Season Voice will be unlocked. If the selected Idol is displayed in the Producer's My Desk Featured Unitx, the Idol will have a special line, depending on the month.