(Since Valentine's day is coming up I decided to do this)

Do they even celebrate Valentines day?

No they don't actually. They want to though but they can't

Why they can't celebrate it?

1)They don't partners/lovers

2)It's not even a holiday in their world so they didn't even know about it until they were reborn as in the human world so they literally don't know what to do

Have they ever even had a partner/lover?

Nope. Never had. Because they're not human they don't want to feel the pain of loosing another loved one

Has anyone send a love letter to them?

Yes! Oh my god they were so popular in school and they still are! They literally get so many presents each year and they don't even know who they are from

Have they ever send love letters to anyone/presents?

In school they had to do that because their teachers told them too. But apart from that they haven't send anyone anything.