Scarlett will need to pickup this weapon while she's in the Twilight World fighting against Princess Chiamaka's Phoenix spirit-form. After Scarlett manages to engage the Princess's spirit-form in melee combat (using her Moonblade), she can dislodge a single feather quill and a scale from the giant spirit-bird. This is the Spike & Scale, which can be used as a Spear and Shield in the rest of the battle. The Scale, when used as a shield, can not only protect Scarlett from Princess Chiamaka's spell attacks, it can reflect the spell right back on the Princess. It is the only weapon Scarlett can use to complete the battle.
After that climactic battle, when Scarlett returns to the mortal world, she may continue to use the Spike & Scale as a Spear and Shield if she wishes; however, after that battle, Scarlett will also have earned the Holy Spear of Chiefs as a reward (from the chest in the Tree of Life), and since that is a far superior Spear weapon, she really won't have much use for the Spike & Scale afterwards.