Ninja Blade
The Ninja Blade (Shinobi Katana in Japan) is a Light Blade found in both Golden Sun games. It can be bought in Lunpa in Golden Sun and in Izumo in The Lost Age. It can be bought for 8800 coins and sold for 6600 coins. Being a Light Blade, the Ninja Blade can be equipped by Isaac, Felix, Garet, Jenna, Ivan, and Piers. It increases the wielder's attack by 94.
The Ninja Blade's Unleashed power is Cyclone Slash (Resist Slash in Japan). Cyclone Slash converts all damage to Jupiter damage and attacks with an additional 38 attack points. It may also lower the target's resistances by 40 (this effect can stack twice). Visually, Cyclone Slash appears as a light violet whirlwind that blasts the target into the air when the blade hits.
This generally becomes Ivan's best weapon for when it can first be found, which is after completing the optional Lunpa Fortress and the shops in Lunpa reopen for business. The blade's Unleash has its power increased if its wielder has a high Jupiter power rating, which typically means Ivan, so he can do respectable damage with the Unleash to enemies that have low Jupiter resistance. The Ninja Blade is not as useful in The Lost Age, though, because the Jupiter Adept in your party there, Sheba, holds Maces instead of Light Blades and can't equip it.