I got Pebble! The unlock condition was fun but I agree with the comment saying he's a bit underpowered, especially when gunning for the achievement (which I understand is a challenge but it seems a bit too dependent on restore cell luck). The -2 stamina on the magma floor was tough even when I used ladders and mageflare runes to get most restore cells. Maybe Pebble can have an extra drink from the flask or have an in-built ability similar to Sanguine Signet? The idea of digging around on purpose as not to overheal is fun but so far it seems quite luck dependent.
Thanks for frequently updating this game! Was exciting to play with a new turtle :) Also appreciate the balancing for Griddle and Fern.