“N-No, it's nothing. Sorry for interrupting your discussion”
“Really? Then that's good. So anyways...”
Koutarou hadn't offered a decent explanation, but the girls were so interested in dieting that they quickly forgot about him.
“... Geez...”
With the girls having lost interest in him, Koutarou let out a sigh of relief and began talking to Alunaya again.
“Sorry for startling you”
“No, it was just so unexpected... So will landlord-san's dieting really fail?”
“Yes. The reason for why Shizuka's weight has increased is me”
Adhering to Koutarou's request, Alunaya began explaining the situation.
Alunaya was so powerful that he had an effect on his surroundings even if he didn't do anything. Since that would be bad, he was normally using magic to neutralize the effects. It was thanks to this that nobody had noticed Alunaya who was inside of Shizuka for all this time. However, having used up the majority of his powers in the previous battle, Alunaya was no longer able to fully neutralize the effects on his surroundings.
”...and that's how it is”
“So landlord-san hasn't gone up in weight, it's just her surroundings that have become weird?”
“That's right. Because I can't correct the distortions in the space surrounding Shizuka it looks like she had gone up in weight”
“So, lowering her weight through dieting is...”
“Impossible. There's no way to lower her weight except waiting for me to regain my powers”
Shizuka hadn't gotten fatter, yet the space around her being distorted caused her weight to increase. Lowering her physical mass would have no effect on the distortion. That was why Alunaya had said that the dieting would fail.
“If we leave it like this, Shizuka will eventually notice the abnormality with her body. She'll probably find out the next time she gets on a scale. What do we do, Blue Knight?”
“T-That's bad! Do something!”
By summoning Alunaya, Koutarou had, strictly speaking, involved the unrelated Shizuka into the battle. He felt that it was his duty to make sure that Shizuka could live her life like normal.
That's why he hadn't told her about Alunaya. Nobody would be happy to learn that there was another creature inside of their body after all. That's why Koutarou needed to keep Alunaya a secret to protect Shizuka's everyday life.