Belath'nin was a Ba'ku who left the planet of the same name in 2275 after joining the Son'a. Sojef mentioned this individual alongside Ro'tin and Gal'na when recounting the Time of Sorrows. (TNG novelization: Insurrection)
Belath'nin is presumably the character's original Ba'ku name, as he was mentioned alongside Ru'afo and Gallatin by their Ba'ku names. As members of the Son'a had renounced their given names in favor of ones taken from historical Ka'bu figures, it is unknown what name Belath'nin chose after joining the Son'a.