Thread:Blue21Phoenix/@comment-26221199-20150318230903/@comment-5314514-20150319020205

Scattered throughout the light novels, any mention of Tet or when he is in a conversation has him referred to as a male (ie "him", "he", etc). Also, in accordance with the 6th novel, it is assumed he was the materialized kid that Riku imagined in his childhood (I have yet to finish light novel 6 so I can't give a definitive answer on that possible evidence). Unfortunately, Yen Press has licenced the light novels so I can't provide you a link to any online translations of them as almost all have been taken down. In addition, subbed versions of the NGNL anime also refer to him as a he (check crunchyroll if you would like to validate it yourself).

Due to the nature of the light novel licensing and the unstable evidence from anime translations even from crunchyroll, I can't provide a link to solid evidence on Tet's gender even though it is clear that he is male in the light novels.