Koitsu (DR1-EN135) - Dark Revelation Volume 1 Unlimited

Once per turn, during your Main Phase, if you control this monster on the field, you can equip it to your Aitsu as an Equip Spell Card, OR change it back to a monster in face-up Attack Position. When equipped to a monster by this card''s effect, that monster''s ATK/DEF is increased by 3000 points, and when it attacks with an ATK that is higher than the DEF of a Defense Position monster, inflict the difference as Battle Damage to your opponent''s Life Points. (1 monster can only be equipped with 1 Union Monster at a time. If the monster that this card is eqipped to is destroyed in battle, this card is destroyed instead.)