It’s the same with babies kinda, their brains are more adaptable to knowledge.
Pre-evolved Pokemons were simple-minded creatures. They only think about 3 things, that was to get stronger, to eat, and to sleep. After they start to evolve, they start to get intelligent. That was why pre-evolved Pokemon tend to learn moves faster and grows stronger at a rapid pace than the evolved Pokemon as its mind only revolves only around 3 things.
Anime & Comics · TheSilverPrince
+1
1) They are simply born in the dimensional gap, but near to the dxd verse and that is why they are not rejected by it.
Anime & Comics · illyisabot
Writing*
1: At least ten minutes of face-to-face confession are required.
Highschool DxD: I have a Dimensional Travel System
Anime & Comics · heavenlydawn_