PLEASE NO SPOILERS FOR ALL IN, I ASSUME ITS ALREADY AIRED FOR SOME Ok so we all know the letter Sprig was able to read. It was blank to the others. Now let's head back to The Core and The King. Leif was able to create a way for Andrias and her to give messages secretly. Andrias used his glasses to see it, but how did Leif see it? We don't know. Maybe some sort of special power she was given. So going off that, here's my theories 1. The power got passed down to Sprig. Say many members of the Plantar family got the power, but never knew until Sprig found that letter. Sprig somehow knew no one else could read it. Maybe his mom or dad knew but told Sprig to keep the power secret? (SPOILERS FOR WARRIORS POWER OF THREE UP AHEAD) 2. Sprig is a Leif reincarnation. Leif and Sprig are SHOCKINGLY similar in design. I know some family members can be similar, but Leif would be over 1000 years old, so she couldn't possibly be alive. But say it's like Cinderpelt and Cinderheart. After Cinderpelt's death, Cinderheart was born. But wait, the Cinder reincarnation happened immediatly after the first's death! That's where Jayfeather comes in. Jayfeather is a reincarnation of a cat from MANY moons ago, Jay's Wing. I only have like 5 minutes until All In airs so I'll sum this up. Leif reincarnated into Sprig. Sprig retained the power and memory of being able to read the messages Have fun watching All In guys :')
Writing this quickly so I apologize for potentially bad grammar :')