Considering the upper-case/lower-case characters corresponding to the 26 ISO basic Latin alphabet letters that your average Modern English speaker would argue any other characters* are instances of: as of Generation VIII, there are 19 such Pokémon that have characters beyond those 26 letters in their English names. They are as follows:
Generation I:
1. Nidoran♀
2. Nidoran♂
3. Farfetch'd
4. Mr. Mime
Generation II:
5. Porygon2
6. Ho-Oh
Generation IV:
7. Mime Jr.
8. Porygon-Z
Generation VI:
9. Flabébé
Generation VII:
10. Type: Null
11. Jangmo-o
12. Hakamo-o
13. Kommo-o
14. Tapu Koko
15. Tapu Lele
16. Tapu Bulu
17. Tapu Fini
Generation VIII:
18. Sirfetch'd
19. Mr. Rime
There are additionally a number of Pokémon whose forms have different characters (Zygarde and Unown, for example).
*on the tangential side-note, a smattering of other non-punctuation characters that can appear in modern English writing: (ë, ï, ö (standard writing) and è, ì, ē (in poetry) can show up in non-imported words; é, æ, œ, ç can show up in imported words that aren't immediately registered as imported; and ü, â, î, ô, and ñ can show up in recognizably imported loanwords)