In releasever.json you can find a mapping of character id to the first version they appeared in.

Notes:

• Release version timestamp does not refer to when the version is released to players. Typically the character is only available several days after the timestamp.
• There is no data for characters before 19-12-28-08-02-55-e463c0 (Few days before Snowsant's release). Public release dates are given instead.
• char_008_owl and char_279_knit3 are not playable characters and will not show up in character_table.json