Mother 2
In Mother 2, The Mr. Saturns seem to be troubled by their friends being kidnapped by Master Belch, Ness decides to go to Belch's Lair (Which has both a weird entrance and password) and free the Mr. Saturns. Back at Saturn Valley They are "Ful-thank" and give Ness and Co. some goodies.
Later, Ness and Co. come to Saturn Valley to find Dr. Andonuts and Apple Kid there, building the Phase Distorter.
 Mother 3
Mr. Saturns first appear when Lucas and Boney (depending on your choice, Kumatora can be there too), find the hidden village overrun with Pigmasks, who also captured Duster. After freeing Duster and fending off the Pigmasks, the Mr. Saturns are grateful to Lucas and company. They show Lucas Fire Mountain where Phyrgia and one of the Seven Needles lies. One Mr. Saturn also borrows Lucas' Courage Badge, which later becomes the Franklin badge after much polishing.
Mr. Saturns are odd creatures, even by Mother standards. The text used when they talk is of a unique font, as opposed to the usual font used by every other character in the game. They also say "boing", "ding" and "zoom" a lot, and seem to like stacking up on top of each other to imitate ladders. Their favourite food are peanut-cheese bars, which, in Mother 2/Earthbound, can heal characters for 100HP. Most Mr. Saturns also appear to be very shy, but make up for it for their superior knowledge... And... Birdie Fishing...