People are downvoting you and aren't even providing a reason for their disagreement.

Spectra has a non-glancing AoE Slow with a chance for ATB Reduction and a skill that does damage based on enemy MAX HP (i.e. good for bosses).

Magic Knights have AoE capabilities on their S1 making Despair a potent choice. Lupinus specifically has a silence on her S2 and her S3 does a multi hit similar to the S1, doing damage based on enemies MAX HP and guarantees ATB pushback activation.

They have similar capabilities, but Spectra shines more due to his S3 not doing damage thus capable of being applied to any element.

Spectra's S2 damage is 3.5xATK + 0.18 MAX HP

Lupinis S3 damage is 3 x (ATK + 0.06 MAX HP)

So it's slightly different.

You'd have to call out specific scenarios if you want to claim that she is better than Spectra. Spectra is mostly used in DB10 and in ToA, both situations where Lupinus would fair worse (against fire and no AoE slow) than Spectra.