OoC: bad news: you can't do any supernatural stuff. Either you come with a logical explanation, and I don't think you can- especially since you never mentioned that sword in your description either.
So either we skip past this whole transformation or you have a reasonable, non-magical explanation.
Also, I believe you mixed up Shikai and a Hollow's Bankai. The Shikai changes the shape of the sword, but it does not fuse it with the user body- no sword in Bleach has done that. They also usually keep the form of a bladed weapon (if they're the physical type) or assume totally random forms if magical type (like Haneneko that becomes sand capable of cutting anything it touches)
Bankai too don't fuse with the user body- barring Ichigo's. They are the spirit of the sword that takes form, and they're usually big. Very big.
Hollow's Bankai however are different- I'm not even sure they're bankai actually.
But why am I making a conference over Bleach? You can't use it anyway.
Hyacinth was still blinded by the sudden flash- when he heard the trainer saying "I caught Grumpig!" Blinking a little, the ranger felt better at Redino's exploit.
"That's good," he said, shaking his head as his vision was still a little blurry. "Say, you want to get out of there, or you want to continue exploring the cave?"
It had now stopped hailing, and the pellets of ice were not sitting on the ground, soon to be melted when the sun would come out of the clouds' screen. The sudden downpour had frightened most Pokemon, that now that it finished, they started slowly flocking back outside- which meant it was no big deal that the patch behind the ranger was rustling. Another Pokemon, maybe?
"Unown," the Pokemon said as it floated out of the grassland. Its big eye surveyed the human interestingly as he asked the trainer what he wanted to do.
On the other side of the spectrum, through, the cave was still very dark as it went on and on- a low growl echoed through the walls, as if calling out something, or someone...
Area effect: Redino is inside a cave. Light filters from behind him, but there isn't much due to the hail clouds shielding the sun. The cave is dark that you can barely see Grumpig. The cave is large enough to let An Aggron comfortably stand, though it'd have problems moving around. The cave seems to extend underground for long.
Genderless Unown (??/ Levitate)
Name: Redino Xenerous
Location: Meteor Valley
Total Items: 8 hyper balls, 9 Max Potions
Outside its Pokeball: Absol(M)(brave) 63.11%
TMs/HMs/BMs/SMs/MTs: TM Stone Edge, TM Shadow Claw, TM X-Scissor, TM Iron Tail, TM Rock Tomb, TM Aerial Ace, TM Giga Impact, TM Protect, TM Fire Blast, TM Flamethrower, TM Hail, TM Ice Beam, TM Blizzard, TM Substitiute, TM Thunderbolt, TM Calm Mind, TM Will-O-Wisp, TM Thunder, TM Dream Eater, TM Sleep Talk, TM Dark Pulse, MT Superpower, MT Icy Wind
TMs/HMs/BMs/SMs/MTs: TM Protect, TM Roost
Rash Poochyena (Quick Feet)
Bashful Grumpig (Thick Fat)
#1 Abra- Fainted
#2 Male Rash Poochyena - Caught
#3 ?? Bashful Grumpig - Caught