In Loveless, fighting requires two people, the fighting unit (also translated just as fighter) and the Sacrifice.
The Sacrifice commands the fighting unit, but takes all the damage.
At the spell battle school which exists in the series, there are implications that fighting units are viewed as less than real people when compared to their Sacrifice counterparts.
Sacrifice/fighting unit pairs are destined to work together.
They share one name, and it is written on both their bodies. (Unclear whether it is always in the same place on both.)
Fighting units can operate on their own ("auto") without a Sacrifice, or with a Sacrifice of a different name, but both decrease their power.
It also seems that operating with a fighting unit of a different name increases a Sacrifices's vulnerability when under Restriction. (See below.)
Most spell fighters view either of these operating methods as distasteful.
Spell casting is based around the power of words. A spell's complexity and power increases the more kanji (Japanese characters) are used in the spell activation.
In addition to the activation word(s), there is also what I call a "support incantation."
Support incantations are spoken after the activation words, and describe the spell's effect more precisely.
The series has shown support incantations to be effective both in shaping a spell to find weaknesses in the word choices of an opponent,
and also as a means of raw clashes of power, where one fighting unit attempts to force their description of spell effects into existence instead of the other's.
Two common spells are Restriction (activation words vary) and Defense. Defense is just what it sounds like, and can use support incantations. Restriction manifests as chains on the opposing Sacrifice (and sometimes also their fighting unit) and increases the damage taken by the Sacrifice.