Junior Tribune was a title given to the group of younger Aquana who sat on the Ruling Tribunal of the Aquans. According to the older Tribunals, they always want to change the ancient records.

Lemus, Nephro, and Rila were Junior Tribunes in 2269. (TAS: "The Ambergris Element")

Though not explicitly stated in dialog, the script notes made it more evident that the division of younger tribunes that were sitting together, and later involved with helping Kirk and Spock, were in fact the Junior Tribune.