My entry in  GrimmInHisCrib's Baby Ashchu month contest!

Story
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"We did it!" Ash says, "With this victory over Alohla, we've beaten every pokemon league currently in existence!"
"I hear they're making a new one."
"Well then we'll just have to beat that one too!" he laugh.
"Whatever you say 'Champ'."
It was quite a while ago that Ash's life had changed forever. Once a human, he was transformed into a pikachu through the use of a strange power known only as Pokemagic. Originally he was told it would only be temporary, but the day ended and he found he was still a pikachu. The week passed as he continued to travel, nothing. He became distressed that it might be permanent, but he managed to calm down from the support of his friends, and his partner Pikachu. When they had arrived at the next town, Ash was worried that now he was a pokemon he couldn't compete in the league anymore. His fears were alleviated however, when he found there was no ruling against it. Furthermore, he himself could join in on the battles! This was great news, as was the surprise his friends had gotten him to cheer him up, a new shirt and jacket, much like his old one, now the proper size for him to store his badges in.
"Hey now! I haven't lost a single battle yet!" Ash defends.
"Heh, you've lost one! Mr. Piddle pants, you've lost your battle with the potty!"
It was also at this time Pikachu started acting a bit weird. He had gotten Ash's old backpack and filled it with diapers, carrying it around with him wherever he went. Pikachu was stronger than Ash, and easily overpowered him, forcing him into the diapers. And would refuse to let Ash take them off well until after they reached the Jhoto region. It was however that by then, Ash had become so used to them he could no longer hold it in, nor make it to the toilet.
"And besides, Pichu," a nickname Ash had been getting recently, "I know how much you like them, I see you poking and playing with them when you think I'm not watching."
Caught red-pawed. Ash blushes, "Y-yeah... so?"
"Nothing, champ~"
"H-hey, can I ask you something?" Ash decided now was the right time. He wanted an answer to a question he's had for so long. "Why... all this? The diapers... the onesie..." he says looking down at his newer outfit, a onesie with the design of his old jacket on it, "C-calling me Pichu... and... why did you decide to finally evolve here?"
Pikachu just laughs, "Well... what can I say champ? I always wanted a baby brother, and while I've yet to find a way to make you the Pichu that you know you really are at heart," as he says this, Ash's tail wags from hearing that, "I decided the next best thing would be to make me bigger. By becoming a Raichu." he smiles. "Plus..." he pauses, checking Ash's soaked diaper using telekinesis like he had done many times before, "The new powers make changing you a lot easier. Now, let's get you cleaned up, piddlechu!"
The two of them share a tender hug, before returning to their normal routine.