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Probably a coincidence, but if you group the Uniters based on their Creature type you can see some similiarities between them:
>Flying type (Lewa and Tahu)
Both have a lot of metallic armor (silver for Lewa, gold for Tahu) compared to their Master forms, leaving little space for their primary color. Both have their torso shell color match their metallic armor.
Both wield twin blades as weapons. Uxar and Ikir have similar builds.
>Defense/armor type (Kopaka and Onua)
Both have a lot more of their main color compared to the other 6. Both have their torso shell match their primary color, the only ones out of the 6.
Both wield weapons with launchers. Melum and Terak have similar looks.
>Attack/tailed type (Gali and Pohatu): both gain a new secondary color compared to their Master forms (orange and dark tan respectively) and use a lot of it in their armor.
Both wield bladed spears with something added to them.
You kind of get the feeling they were designed in pairs. Also, if you pick one Toa per pair you can form the two G1 Kaita trios, but I doubt that was on purpose.
