If you use your wind team much, Oberon. The 2nd skill improvement for Oberon is enhancing the defense debuff from -10% to -15%.

If we get the Koihime Musou collab in 1 or 1.5 months, then the corresponding gacha would have Sun Quan at SR. She's a Fire kamihime whose 2nd skill confers regen to the party. That upgrades from a total of 750 HP every 8 turns to 900 HP every 8 turns. And her first skill is a defense buff.
Although if you already have Ares and Svalog to bash faces in, you're probably running Nergal and Baphomet in the last two main slots just for their A-frame -10% debuffs (unless you're using Joan of Arc's A-frame -20% atk debuff or Gawain's A-frame -20% def debuff...). Then it's debatable if putting in someone like Sun Quan would improve the party over either a R tier -10% atk debuffer or a -10% def debuffer.