On the Japanese website for Star Fox 64, Slippy mentions certain background characters who would appear in future installments:

• Peppy's wife, Vivian Hare. According to Slippy, Peppy took her to planet Zoness for their honeymoon, which explains why Peppy is so distraught at its polluted status in SF64. Vivian would appear in-game for the first time in Star Fox Command, where it's revealed that she and Peppy had a daughter named Lucy, and that Vivian had died of a disease several years prior.

• Slippy's father, Beltino Toad. According to Slippy, Beltino works as an engineer for Space Dynamics, the manufacturer of the Arwing and many of the other vehicles the Star Fox team utilize. Beltino would later appear in-game for the first time in Star Fox Assault, and would later appear in Star Fox Command.

It's worth pointing that unlike Vivian, Beltino was mentioned in western ancillary SF64 material, that being both the official western Star Fox 64 website and the Star Fox 64 Official Player's Guide, where it states that he helped his son Slippy create the Blue Marine.