Participates in the primary piRNA biogenesis pathway and is required during spermatogenesis to repress transposable elements and prevent their mobilization, which is essential for the germline integrity (PubMed:23970546, PubMed:26919431). The piRNA metabolic process mediates the repression of transposable elements during meiosis by forming complexes composed of piRNAs and Piwi proteins (Siwi or Ago3) and govern the methylation and subsequent repression of transposons (PubMed:23970546, PubMed:26919431). Required for the final steps of primary piRNA biogenesis by participating in the processing of 31-37 nt intermediates into mature piRNAs: acts by recruiting the exonuclease PNLDC1/trimmer to Siwi-bound pre-piRNAs (PubMed:26919431). Interacts with (symmetrically methylated) Siwi (PubMed:23970546, PubMed:26919431). Interacts with (symmetrically methylated) Ago3 (PubMed:23970546). Interacts with PNLDC1/trimmer; interaction takes place on the mitochondrial surface and recruits PNLDC1/trimmer to Siwi-bound pre-piRNAs (PubMed:26919431). Localizes to the outer surface of mitochondria. The Tudor domain specifically recognizes and binds symmetrically methylated Siwi and Ago3. Belongs to the Tdrkh family.