Here, not only can Tsubaki transform into Black☆Star, she can even move and fight like him also, with the similar amount of strength as her Meister, due to Black☆Star's Soul Wavelength giving her extra strength. However, she is still weaker than him, and thus usually uses this form in order for stealth attacks before assuming another Weapon form.
This form is first used in the final fight against Mifune. After Mifune intercepts Black☆Star's Severed Shadow technique, Tsubaki quickly assumes this form to deal Mifune a hit, distracting him so that Black☆Star can quickly hit him with a World Destruction Cannon. Mifune is impressed that Tsubaki can fight very effectively on her own. However, Tsubaki knows that, despite being able to be stronger through borrowing Black☆Star's Wavelength, she will not stand a chance against Mifune. She transforms back into Severed Shadow to wait for another chance to attack.