Hiding your identity

As in, 'I'm actually your best friend chatting you up under a different username', not as in 'I'm actually a spy seducing you to get information out of you'. They're hiding because they feel they have to, not because they're playing tricks. They're hiding that they're actually a long lost ex, not that they're actually a vampire. They're hiding a pre-established personal relationship (I'm actually your ex's twin, not your ex), not a category they fall into (e.g. merman, assassin.)

For shows where characters hide that they're actually a monster or have superpowers, see this list: link.

For gays hiding malicious motivations for seduction and then accidentally falling for their target anyways, see this list: link.

For shows where both character agree the relationship is fake, even though one or both might already have feelings for the other, see this list: link.

For shows where characters are hiding that they come from the future and are here for a romantic do-over, see this list: link.

Descriptions below don't spoil beyond the first episode