I mulled on this for a while because I wasn't sure how to start it, but I ended up writing more than I expected!

“Are you really sure about this?”
Nana couldn't believe it. She was ready to fully quit being a school idol so she didn't hurt anyone else, and yet this twin-tailed girl smiling in front of her had managed to completely change her mind. Yu Takasaki had convinced her solely on the basis that Yu wanted to see her perform, and to see her be happy, practically confessing her love to Setsuna. "Are you saying it's okay for me to be selfish... For me to see what I love through to the end?"

"Of course!" Yu replied, smiling brightly.

That did it. There was no way Nana could ignore such a sincere expression of love. “Do you know what you just did?” Nana asked Yu, sounding confident as she walked past. “You actually said something much more profound than you probably realize." Facing away from Yu, Nana took off her glasses and undid her braids, letting her hair flow in the wind. The girl who turned back wasn't Nana Nakagawa anymore, but the school idol Setsuna Yuki, back from retirement. “Don't blame me for what happens la-” RIIIIIP-SNAP

The joyous surprise that Yu felt upon seeing Setsuna Yuki return, was replaced by a completely shocked surprise as Setsuna's body expanded out, with an orb of flesh bursting out the front of her clothes. In an instant Setsuna plumped up as her hips widened upon thick thighs, and her breasts popped off the buttons of her shirt, her strained bra just barely holding them back from freedom. Yu thought that Nana's clothes already looked a little tighter than when she first met her in front of the club room door, but she could not have expected this. “S-Setsuna-chan, w-what-?”

A storm of scarlet crossed Setsuna's face as she desperately pulled at the torn garments to contain herself once more. “C-crap! I-I forgot the girdle was close to failing, nngh, but I, mmn, thought the extra corset would help... Why'd it have to bust now...!” Setsuna muttered to herself, ignoring everyone staring at her, which included her former club members that fell in shock from their hiding place behind the corner of the building. A beige bob-haired girl pulled herself out from under the pile and ran over to Setsuna, which was impressive considering her jaw was on the floor.

“SETSUNA-SENPAI!” Screamed out Kasumi as she got to Setsuna, quickly sinking her hands into her senpai's gut to confirm it was real. “W-wha, h-how, did you get so FAT?”

“Kasumi-san! Don't touch me like that!” replied Setsuna as she forced Kasumi's hands off and replaced them with her own hands to soothe her belly after the unexpected groping. “I hid this for so long I forgot how big I was, or rather, I didn't realize how big I'd gotten...” Resigned to the fact that she wouldn't cover herself up, Setsuna waited for her fellow idols and Yu's childhood friend to pick themselves off the roof and walk over before explaining herself. “It's uh, really quite simple...” Setsuna started, “After I quit being an idol, I thought I could relax my diet and eat whatever I wanted, trying out all the snacks I had to previously turn down to keep my figure. I quickly found out I loved eating food, and it spiraled out from there.” Another flash of red showed on Setsuna's face as she admitted, “I even sneaked back into the club room to eat all of the remaining snacks after meeting Yu-san and Ayumu-san in front of it...”

“So that's where they all went...” Kasumi thought as she connected the dots in her head. “B-but still, even if you were shoveling food into your mouth, how did you end up gaining this much weight so fast?” She asked as another attempt to touch Setsuna's belly was thwarted.

Despite feeling embarrassed at her own gluttony, Setsuna lowered her eyes at Kasumi. “Well all those calorie-dense koppepan you put on my desk certainly helped...”

Everyone's eyes turned towards Kasumi, while Kasumi's widened as she realized what she had done. “Ahaha, I uh, just wanted to punish the Student Council President for shutting down the club! I thought I'd make her a little chubby, and I only kept giving you koppepan because I didn't see it working!”

Yu came to Kasumi's rescue, butting in before anyone could question Kasumi further. “Kasumi has a point there... I couldn't notice at all that you'd gotten so big! If you weren't planning on being an idol again, why were you hiding your body?”

“Because isn't it weird?” Setsuna felt ashamed at herself. “Suddenly eating so much and becoming so... large... People don't like that, they'd think I'm selfish...” After a brief silence, Setsuna heard a chuckle coming from Yu. “What? Why are you laughing?”

Yu stopped her laughter, calmly looking into Setsuna's eyes and avoiding her belly to hold Setsuna's hands in hers. “Don't you remember what I told you earlier? It's okay if you want to be selfish! You said yourself that you discovered you love eating, so I don't want you to deny yourself that as well!”

There was no way. There was no way Yu had managed to convince her twice in a row, and yet, Setsuna felt her shame lift off her. “Are you saying it's okay for me to continue eating so much... For me to be this fat?”

"Of course!" Yu replied once again with that smile of hers, before adding on, “Seeing you this big is pretty thrilling... a-as a school idol of course!” Yu shook her hands in front of her to ward of any accusations, but what she said still prompted a head tilt from her childhood friend Ayumu.

Smiling confidently, Setsuna walked away from the group, making her body jiggle as she turned back towards Yu, pumping her fist out. “Don't blame me if I gain more weight!” She said aloud, before turning towards the railing and preparing to sing to all the students outside.

“This song is for a new beginning!”