Individuals whose songs resonate with a relic are capable of producing the "chant" necessary to activate the Symphogear—they are called "Harmonizers." Currently there are three categories of Harmonizers.
The first type are individuals like Tsubasa, Chris and Serena. They are born with an innate aptitude which can stabilize through proper training.
The second type are individuals like Kanade, Maria, Shirabe and Kirika. Although they are born with an innate aptitude, training alone is not enough and they require doses of drugs in order to temporarily raise their link coefficient.
The third type is unique to a single individual, Hibiki. At first she became an Adaptor as a result of her body fusing with her relic after an accident. However, in Earth's eleventh hour, she became a true Harmonizer not through training or drugs, but by forcibly obtaining the necessary link coefficient.
Her transformation from a Relic Fusion to a Harmonizer is still a complete mystery. Despite attempts to research it, there has been little progress, and currently it is simply considered a miracle.