Milking the cow

This quest starts when Lust Doll tries (and fails) to milk Bell at least once. Note that only initial stage is available right away; subsequent parts requires some additional quests being done before it can be advanced.

Succubi magic
If Rinny was previous rescued and Lust Doll learned about succubi body workshop, then, after failing to milk Bell, they may suggest Bell to try it and enlarge her chest there.

But not even the body workshop would work: the moment chest grows up, some unknown force immediately shrinks it back! Rinny, sad because of such failure, presumes that someone must have cursed Bell at some point, and promises to do some research on that matter. She also teaches Lust Doll the spell-growing spell, just in case.

Darker magic
After trying (and failing) to solve Bell's problem with Rinny's help, you may suggest to visit Pixie instead (requires finishing Manor quest first), as she's a powerful mage.

However, despite all efforts, Pixie wouldn't be able to achieve results either, only confirming that it's indeed a curse. If not even Pixie can do it, then who can?..

Holy magic
After failing with dark magic, Lust Doll would remember another option -- the magic bathhouse, set up where the Temple of Polkapola once was; the blessed water, apparently, can cure any curse, likely including Bell's.

Bell is sceptical, still remembering how first two attempts went, but agrees to give it a try... only to wander off almost immediately. Fortunately, a kitsune saw her walking towards the baths to the left. Unfortunately, Bell managed to lock herself inside, and the door is jammed. A kitsune standing there would mention that there's some trick to unlock the door, but it's only known by one of the nekos.

The neko in question can be found on the second floor, standing by alone and staring into wall. Despite the red text warning, it's perfectly safe to approach her. She would help with freeing Bell, and you two would take the bath without problems.

The magic water indeed works... for about ten seconds, as when Bell leaves the water, her boobs quickly shrinks back. As explained by a kitsune, apparently the curse is indeed very strong, and thus, the magic water must be applied permanently, or effect would wear off. But, not all hope is lost: there's a legend about some ancient king's spring; its water is even stronger, so if anything can lift the curse, it's it.

Now, the course of action is clear: find that ancient spring, and bring Bell its water. But where to look for it?