Tandem

Tandem is a mechanic which requires you to have several (2+) of certain characters in the team. If at least one of them gets a TND orb, and if you attack without missing with all tandem characters, a tandem will activate and do something (extra damage, boost).

Unfortunately, tandems are considered weak, because the effects usually are weak and/or require use of weak characters that don't fit better strategies and as a general rule of thumbs, benefits of tandems do not outweight the cost of using suboptimal characters. The only tandem that was recognized as somewhat useful by experienced players was a Marine grunt tandem that can generate fixed damage to kill the Turtles.

Tandems are also present only on FTP characters, with the common explanation being that something on Japanese anti-gambling/pachinko laws that prevents giving rewards that require other parts to assemble into a complete part translates into an interpretation that OPTC, a video games that use micro-payments cannot offer non-F2P characters that require other non-F2P characters to combo with as explicitly as it they had a set tandem. So, basically, until Japanese law changes, tandems will be a marginal and sucky part of the game. If you want to do something about it, complain to the Japanese parliament?