Yamamoto drew a picture of Princess Yukiko of the yokai anime Dororon Enma-kun and Botan a guide to the Underworld of YuYu Hakusho.
Princess Yukiko was his favorite female character from childhood and YuYu Hakusho was he used to practice drawing, and he unexpectedly discovered that they had thing in common with his favorite Pokemon Froslass (ice/ghost) (But isn't your favorite Pokemon Tsareena? XD)
And Ultima said she also wanted to join the pretty girls lol
Source
Yamamoto said he used to be hesitant to draw female characters, but later he decided to refer to the magical girl anime Minky Momo to draw them, which became the basis for his subsequent art style. Then, as Yamamoto had said before, during the selection process for the illustrator of Pokespe, Kusaka immediately noticed that his art style was influenced by Minky Momo. While Yamamoto said that he didn't actually particularly like Minky Momo anime.
Minky Momo
He also said that because he wasn't involved in the creation process of Red and Yellow, his understanding of them was limited, and he didn't quite understand what readers liked about them, making it more difficult to create them. Compared to Scarlet, who was created as a cool-looking character, it was easier to draw her.
Then he said the interactions between Crys and Gold are inspired by the male and female protagonists of Dororon Enma-kun. Originally, the protagonists were the same height in the first anime, but in later versions, the height of the female protagonist Princess Yukiko was increased (a similar change was made in GeGeGe no Kitaro anime). (While in the last version released more than ten years ago, Princess Yukiko's art style changed to the typical moe style.)
Btw, Yamamoto alluded to this by saying, since Gold and Crys have corresponding characters in Dororon Enma-kun, what about Silver? lol
Yamamoto also talked a lot on Twitter about his life before becoming a manga artist. He mentioned that he almost gave up on becoming a manga artist and that he drew some fan mangas back then. It was quite a long post, so I guess he's almost finished with his work right? XD