Partner shuffle

A Partner shuffle (パートナーシャッフル, Pātonāshaffuru) is the option for parasites who experience incompatibility with their partners to switch partners with someone else in their unit.

Overview
Parasites are accordingly paired in boy-girl pairs upon their discharge from the Garden based on their aptitude test results. However, these arrangements are found to be flawed, as it is common for pairs to experience incompatibility in the beginning of their partnership, but it is shown that pairs can experience long-term incompatibility if they don’t share the emotional bond with one other, which is significant for their paracapacity. According to Alpha in the manga, the idea of changing partners is considered impossible. However, this is shown to be untrue, as changing partners, even if temporary, is done as a last resort should a squad be short-handed or at a disadvantage.

Squad 13
It was offered as an option for Mitsuru and Ikuno by Nana when it was found the source of their incompatibility was due to Mitsuru’s unstable yellow blood cell count and this made him eligible for pruning (removed from Squad 13 and eliminated). As she had cared for the parasites of Squad 13 since their infancy, Nana is hinted to have offered the partner shuffle out of affection for the children.

Mitsuru and Ikuno both agreed to the partner shuffle and the rest of the squad convened on whether to switch partners for the two or not. Ikuno asked to try a pistil-pistil pairing with Ichigo, whereas Kokoro asked to try pairing with Mitsuru. In the connection test, it was found that Ikuno and Ichigo had failed to connect in Delphinium, whereas Mitsuru and Kokoro established a stable connection in Genista. This resulted in Kokoro switching partners from Futoshi to Mitsuru, and Ikuno pairing with Futoshi.

The partner shuffle was shown to have successful results. Ikuno and Futoshi’s battle performance drastically improved after they realized they had similar dilemmas (unreciprocated feelings for Ichigo and Kokoro respectively) and this made them compatible. Whereas, Mitsuru and Kokoro’s paracapacity rose all the way to the maximum, far higher than with their previous partners, after Kokoro got Mitsuru to admit the origin of his grudge against Hiro and she comforted him.