A potential girl character that could be like the MC while in the same dimension could be Angie who was raised at a day care center and trained a Licktung into a Lickilicky where she hide her in a cave. So it can go that the MC heard of a scarf monster and wanted to investigate if its a new legend or something. He finds both Angie and Lickilicky together and she does not want her parents to find out so she decides to make him orgasm to keep him quiet. When she sees his dick change into a pokemon she would show she also can change parts of her body but mostly her tounge and decides to have sex with him to reduce some stress. Lickilicky decides to join in on the fun with her massive tongue and both end up being inflated and ask him if he wanted to come back for more fun latter. The two would go to the cave for some tounge training which is them 69 each other.
The idea of the alternate dimension female MC sounds perfect for that part. Will the MC get to try his alternate dimension female self breast milk like the Ultra beasts milk?