What would be the worst devil fruit for your character to have, thematically or power wise?"
For Yasashī it would be the Leopard Fruit. He already struggles to contain his bloodlust, and a carnivorous Zoan would make that ten times worse.
Whenever he looses control he transforms into a sulong-like beast, and the Zoan would be added on top of that.
Also, he is already fussy about his fur, because the red colour reminds him of the blood on his hands.
The leopard Zoan would make his
"Out damned spot" tendencies literal.
If this were Marvel there would be a "What If" bad timeline where he joined the Cipher Pol and got the leopard Zoan.
His fur colors all represent different aspects, red is blood and aggression, black is repression and white is the rest (feline instincts and stuff). Should it ever exist, even though it is a carnivorous Zoan, the White Tiger fruit would be interesting.