Tenacity Increase is buff that increases hero's chance to resist debuffs, for example Daze, Bleeding, Blindness etc.
While in the battle, you can tap and hold the Buff icon and read what it does.
Here's a list of heroes capable of casting Tenacity Increase:
  • Eric Shieldbreaker (Ability 3)
  • Niya (Ability 3)
  • Solius (Ability 3)
  • Rantha (Passive Ability)
  • Mortha (Ability 3)
  • Tromgar (Passive Ability)
  • Kelrian (Passive Ability and Ability 2)
  • Instructor Gorram (Passive Ability)
  • McRat (Ability 2)
  • Patriarch Qi (Ability 3)
  • Hilda (Ability 2)
  • Lake (Passive Ability)
  • Kaggie (Passive Ability)
  • Salvador (Ability 2)
  • Nessa (Leadership Ability)
  • Rawley (Ability 2)
  • Hell Kitty (Ability 3)
  • Baldur (Ability 3)
  • Ambu & Lance (Ability 3)
  • Bar & Rel (Passive ability)
  • Kaira (Ability 3)
  • Megawheel (Ability 3 - Tourment)
  • Shaiya (Ability 3)
  • Buff (Ability 2)