You can equip one item at a time which gives you different buffs depending on the item but it won't act as a cosmetic.
A few examples
Dmg booster
Reverse card- Boosts the dmg of elemental Jutsu and decreases the dmg of non-elemental Jutsu. Nothing happens if the user does not have non-elemental mastery.
Elemental seal- Decreases dmg of elemental Jutsu while increasing dmg on non-elemental mastery. Nothing happens if the user does not have elemental mastery.
Flame orb- Increase dmg of fire Jutsu but decrease dmg of any other mastery moves.
Archer medal- Increase dmg caused by throwing tools such as Senbon, shuriken, and kunai.
Items already in the game
Honey- all mobs aggro you even if they are usually peaceful.
Kazecola- Increase hp regeneration speed.
Items affecting other factors
Shiny stone- Increases the drop rate of items when held.
Secret scroll- Leaves the user at one hp when hit by an attack that will kill. One-time use only unless users die or get back to full hp does not ignore damage over time.
Expensive Smog bomb- Increases the time cloak is active by 7 seconds.
Sensei letter- Increase the dmg of everyJutsu when the user is below 20% hp.
Troll items
Final wish - Shows a emote when the user dies.