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STR or Strength affects the damage that physical attacks deal.
DEF or Defense affects the damage you take from enemy attacks.
SPR or Spirit affects the effectiveness of your Yo-kai's Technique move.
SPD or Speed affects the turn order and the number of turns the Yo-kai get each battle.
Other Stats
HP reaches 0 and the Yo-kai is KO'd.
Soul Meter is full then players can activate the Yo-kai's Soultimate.
Rank ranges from E, D, C, B, A, S. Higher the rank equals more powerful Yo-kai.