Wind
Wind is a flexible element, with many skills that increase attack frequency and the Burst gauge, allowing for more frequent Bursts. However, Wind has few options for healing until much later. Wind is particularly friendly to free players, as you get immediate access to a strong type B debuff from Cybele, and the easily-obtainable Zephyr (R) adds weak heals and an amazing type C ATK debuff. This makes Wind more flexible than other elements, allowing room to change Souls and EX abilities to fit the encounter.
Wind starts somewhat weak but strengthens with later content updates. Gaia’s Awakening is the turning point for Wind, and makes her the single must-have character for Wind. She’s the first character to get an Awakening, and she becomes the Paladin of KamiPro, uniquely able to nullify attacks on other characters, mitigate AoE overdrives, AND heal the team. Cybele’s SSR variant transforms her into a peerless attacker GUARANTEED to combo with every normal attack, giving her ridiculous Burst generation and damage output, especially when buffed (e.g., by Morrigan and/or Titania).
Wind’s type A debuff options are somewhat limited, and Wind teams cannot reach the defense debuff cap through normal means. Put another way, Wind will benefit from any DEF debuff that stacks with type A and type B. That means that bringing in an off-element kamihime like Cthulhu with a type C DEF debuff will actually INCREASE damage (assuming it lands!). This adds flexibility even for highly-optimized teams, especially since Cthulhu also brings a type A ATK debuff. Failing that, Oberon (SR) packs both type A ATK and DEF debuffs in one character, and is a solid option until Haster becomes available. Oberon is also the only option for a type A ATK debuff, other than from one’s Soul, which may make her worth bringing along regardless.