Savannah. AND I found out in the game in a funny way...
...so, in City Folk, the way you "set" your birthday was a villager would ask you. Savannah asked me, I told her. She immediately was like "OMG we're BIRTHDAY TWINS! We should have a joint party! Etc." At first I thought the chance of us ACTUALLY sharing a birthday were so slim... there was just no way. And I remember thinking "Well, that's dumb. Why would have that dialogue in there since every villager actually has a birthday? What if they say that but they don't REALLY share a birthday?" Then I looked it up. Turns out, they just had easter egg dialogue in case you DID share a birthday with a villager!
I share a birthday with Rosie, so as adorable as she is, I won't have her around on my island around that time of year. XD My S/O recently brought Rosie onto his island via Amiibo and she will be one of his permanent residents, so I'll be able to celebrate with her without having her steal my thunder!
Wait... does sharing a birthday with a villager effect your celebration in any way? I can't imagine it would- the event for celebrating your birthday is totally separate from the event for celebrating a villager's birthday.
I mean, I'd take Savannah on my island at least for a bit- she's cute.
So real life birthday I share with Cleo and Flora. Cleo is a sweetie, I have not had Flora so I'm not familiar with her.
Luka, my New Horizons character, shares a birthday with Naomi. Another villager I am not familiar with.
June! Which I always find funny since her English name is June but her birthday is in May. She's cute but I've never had her in any of my towns on any game, actually.
Pecan the squirrel. I actually ran into her a lot and thought about taking her but I'm really determine to find Marshal so I go around 50 tickets before I settle.