Contract for a shinyuu partnership.

Shinyuu, sometimes translated as Sister-in-Arms, is a term unique to Tenchi Academy's Hoshitori system. It is pronounced identically to 親友 (shinyuu), "intimate friend," but written as 刃友 (shinyuu), "blade-comrade."

A shinyuu is a sword-bearing student's partner. A shinyuu pair will fight as a team to steal other students' stars. Shinyuu association is exclusive, and is fixed by a contract and by the exchange of Pair Rings, though it is possible to annul a partnership. The shinyuu relationship has parallels to marriage, and is often spoken of in similar terms.

Though swordbearers without a shinyuu are permitted (in fact, obligated) to participate in the Hoshitori, none of the stars they win are considered valid, and they receive neither the improvement in rank nor the material rewards associated with them. However, stars they lose are valid, and are counted against them. As there is no benefit, only detriment, to fighting duels, most solo swordbearers make every attempt to avoid them, running away or hiding if necessary.